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Recommendation to Groups


Abstract and Figures

Recommender systems have traditionally recommended items to individual users, but there has recently been a proliferation of recommenders that address their recommendations to groups of users. The shift of focus from an individual to a group makes more of a difference than one might at first expect. This chapter discusses the most important new issues that arise, organizing them in terms of four subtasks that can or must be dealt with by a group recommender: 1. acquiring information about the user’s preferences; 2. generating recommendations; 3. explaining recommendations; and 4. helping users to settle on a final decision. For each issue, we discuss how it has been dealt with in existing group recommender systems and what open questions call for further research.
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Recommendation to Groups
Anthony Jameson1and Barry Smyth2
1DFKI, German Research Center for Artificial Intelligence
2Department of Computer Science, University College Dublin
Abstract. Recommender systems have traditionally recommended items to in-
dividual users, but there has recently been a proliferation of recommenders that
address their recommendations to groups of users. The shift of focus from an
individual to a group makes more of a difference than one might at first expect.
This chapter discusses the most important new issues that arise, organizing them
in terms of four subtasks that can or must be dealt with by a group recommender:
1. acquiring information about the user’s preferences; 2. generating recommen-
dations; 3. explaining recommendations; and 4. helping users to settle on a final
decision. For each issue, we discuss how it has been dealt with in existing group
recommender systems and what open questions call for further research.
20.1 Introduction
Almost all of the techniques of web-based personalization discussed in the other chap-
ters of this book are designed to allow effective adaptation to individual users. But often
the users of such systems operate not individually but in groups, which may vary from
formally established, long-term groups to ad hoc collections of individuals who use a
system together on a particular occasion. This phenomenon can in principle occur with
just about any form of web personalization. In this chapter, we will focus on the sub-
class of recommender systems (cf. the chapters in this volume by Schafer et al. [34];
Pazzani & Billsus [31]; Smyth [35]; Burke [5]; and Goy & Ardissono [15]), but many
of the points made will be applicable by analogy to other types of adaptive web-based
system (cf. Section 20.6.2).
Some types of items that a system can recommend (e.g., restaurants and museum
exhibits; see Table 20.1 for additional examples) tend to be used at least as often by
groups as by individuals, so addressing recommendations to individuals can actually be
unnatural. Moreover, the evolution of computers away from the desktop PC makes it
increasingly natural for systems to address groups as well as individuals: Wall displays,
information kiosks, PDAs, and cell phones can be used easily by persons who are in-
teracting with each other. And even with the traditional PC, users are being offered an
increasing variety of ways to communicate with each other and perform tasks together.
For these reasons, we can expect a continuing growth in the trend toward recommenda-
tion (and, more generally, adaptation) to groups of users.
In this chapter, we will identify the issues that should be addressed by designers
of group recommender systems and the ways in which they have been dealt with in
P. Brusilovsky, A. Kobsa, and W. Nejdl (Eds.): The Adaptive Web, LNCS 4321, pp. 596–627, 2007.
Springer-Verlag Berlin Heidelberg 2007
20 Recommendation to Groups 597
systems that have been developed so far. Our two goals are to allow designers and
researchers (a) to make effective use of knowledge and experience that have already
been accumulated and (b) to address the many open questions that still require careful
consideration and research.
20.1.1 Existing Group Recommenders
Figure 20.1 lists almost all of the group recommender systems that, according to the
authors’ knowledge, have been described in the literature up to the time of the writing
of this chapter. Although a number of these systems have been described only briefly
and some were not implemented as web-based systems, we will refer to aspects of all
of them, so as to convey an idea of the variety of application settings, design issues, and
possible methods that designers of group recommender systems should be aware of.
20.1.2 Overview of Recommendation Subtasks and Issues
Relative to recommendation for individuals, there are a number of new issues that arise
with group recommenders. Table 20.2 organizes these issues in terms of four high-
level subtasks than must (or can) be performed by a group recommender. The issues
corresponding to each of these subtasks will be addressed in one section of this chapter.
These subtasks differ greatly in the amount of attention they have attracted in re-
search so far and hence in the length of the corresponding sections of this chapter. By
far the most research has been done on Subtask 2, a fact that is understandable given that
any group recommender must have some way of assessing the suitability of items for
the group. Subtask 3 is the second most popular one, especially given the growing inter-
est in making the reasoning underlying recommendations comprehensible to users (cf.,
e.g., the chapter in this volume by Schafer et al. [34]). Subtask 1 has attracted much less
attention, because many methods for acquiring information about users’ preferences are
equally applicable to groups and to individuals; though the extension to groups does not
always require a change in methods, it can create opportunities to introduce new meth-
ods. Subtask 4 has attracted the least attention of all, since it is usually assumed that
the final decision about whether to accept a recommendation will be made by a single
group member or in face-to-face discussion among group members.
20.2 Acquiring Information About Group Members’ Preferences
Most group recommenders developed so far apply methods for acquiring information
about users’ preferences that are barely distinguishable from the methods applied in rec-
ommender systems for individuals. After briefly surveying some typical applications of
such methods, we will look at preference acquisition methods that have been developed
specifically for group recommendation settings.
598 A. Jameson and B. Smyth
Table 20.1. Overview of the group recommender systems mentioned in this chapter.
System Reference (Examples of)
Groups of Users Items recommended
Web / news pages
Let’s Browse Lieberman et al.
(1999) Persons browsing the web
together Web pages
G.A.I.N Pizzutilo et al.
(2005) Persons viewing a wall
display or information kiosk News items
I−Spy Smyth et al. (2005) Employees of a company Web pages
Tourist attractions
Intrigue Ardissono et al.
(2003) Tourists Sightseing tours
CATS McCarthy et al.
(2006) Friends planning a vacation Vacation packages
Travel Decision
Forum Jameson (2004) Friends planning a vacation Criteria for choosing a
vacation package
Group Modeler Kay and Niu
(2005) Persons visiting a museum
together Information about
RestaurantFinder McCarthy (2002) Colleagues going out to dine
together Restaurants
Music tracks
MusicFX McCarthy and
Anagnost (1998) Persons working out in a gym Music stations
Flytrap Crossen et al.
(2002) Persons using a public area of
a building Music tracks to be
Yu et al. (2005) Passengers in a vehicle Multimedia items to be
Adaptive Radio Chao et al. (2005) Colleagues working together
in an office Songs to be played on
the radio
Television programs and movies
FIT Goren−Bar and
Glinansky (2002) Family members watching
TV together TV programs
TV program
recommender Yu et al. (2006) TV viewers Sequences of TV
PolyLens O’Connor et al.
(2001) Persons planning to go to a
movie together Movies
20 Recommendation to Groups 599
Table 20.2. Overview of the issues to be addressed in this chapter, organized in terms of four
subtasks of a group recommender system.
Subtask of the
recommender system Difference from
recommendation to
General issues raised
1. The system acquires
information about the
If members specify their
preferences explicitly, it may
be desirable for them to be
able to examine each other’s
preference specifications.
What benefits and drawbacks
can such examination have,
and how can it be supported
by the system?
2. The system generates
recommendations. Some procedure for
predicting the suitability of
items for a group as a whole
must be applied.
What conditions might such a
procedure be required to
fulfill; and what kinds of
procedure tend to fulfill these
3. The system presents
recommendations to
the members.
The (possibly different)
suitability of a solution for
the individual members
becomes an important aspect
of a solution.
How can relevant information
about suitability for
individual members be
presented effectively?
4. The system helps the
members arrive at a
consensus about
recommendation (if
any) to accept.
The final decision is not
necessarily made by a single
person; negotiation may be
How can the system facilitate
the necessary communication
among group members?
20.2.1 Preference Acquisition Methods That Are Not Specifically Adapted to
Group Recommendation
Acquisition of Preferences Without Explicit Specification. As we have seen in the
chapters by Schafer et al. [34]; Pazzani & Billsus [31]; Smyth [35]; Burke [5]; and Goy
& Ardissono [15], many recommender systems do not require their users to specify
their preferences explicitly. With group recommenders as well, it may be possible for
the system to get by with implicitly acquired information about users. A straightforward
example is found in the system FLY TRAP (Crossen et al. [11]), which selects music for
playing in a public room. The system learns about the music preferences of the potential
users by (a) noticing what MP3 files each user plays on his or her own computer and
(b) consulting available information about the music played to derive a model of each
user’s preferences.
Another example is found in LET’s B ROWSE (Lieberman et al. [23]), which recom-
mends web pages to a group of two or more persons who are browsing the web together.
The system makes initial estimates of the interests of its users by analyzing the words
that occur in each user’s web homepage. During the actual group browsing, it analyzes
the words that occur in the pages visited by the group.
600 A. Jameson and B. Smyth
Explicit Preference Specification. But there are some types of group recommender
that do require an explicit specification of preferences. An example is the POCKET
RESTAURANTFINDER (McCarthy [26]), which helps a group of people who are prepar-
ing to go out to eat together in selecting a restaurant. For each of 15 types of cuisine that
might be represented at a given restaurant, each user must indicate his or her preference
on a 5-point scale ranging from “Definitely don’t want ...” to “Denitely want ...”.
Similar ratings are given for 17 possible restaurant amenities, 3 price categories, and 3
ranges of travel time from the current location. (Users presumably consider this rating
effort worthwhile only if they intend to use the system repeatedly.)
Similarly, explicit preference specifications are required by the TRAVEL DECISION
FORUM (Jameson [18]; Jameson et al. [19]), which helps a group of users to agree on
the desired attributes of a vacation that they are planning to take together. The system
needs to know how each user feels about dozens of attributes of vacation destinations,
ranging from the facilities that are available in their rooms to the sightseeing attractions
that are available in the surrounding area. Here again, only explicit elicitation is likely
to be feasible.
A less explicit form of preference specification is found in POLYLENS, an extension
of the MOVIELENS system (cf. the chapter in this volume by Schafer et al. [34]) that
recommends movies to groups of users. Since POLYLENS (like MOVIELENS) is based
on collaborative filtering, users do not explicitly describe their movie preferences, but
they do rate individual movies on a scale from 1 to 5 stars. As we will see, this procedure
raises some of the same issues as the explicit specification of general preferences.
An intermediate case between implicit and explicit preference specification is found
in the system I-SPY (see, e.g., Smyth et al. [37]), a community-based web search engine
that personalizes search results for a community of like-minded searchers on the basis
of a model of community search preferences.1The I-SPY user indicates interest in a
given search result by selecting the result in question from a query result list, and I-
SPY interprets each result selection as an indication of relevance with respect to the
current search query. The specification is implicit in that the user’s primary intent in
selecting a result is not in general to indicate his or her preferences to the system; but it
has some elements of explicitness in that users are aware of the fact that their selections
are being interpreted as reflecting their preferences and can, if they like, choose results
that they would not otherwise have chosen, in order to influence the system’s preference
model (cf. the discussion of manipulation in Section 20.3.2).
20.2.2 Adapting Preference Specification to the Requirements of Group
Focus on Negative Preferences. In the context of the system ADAPTIVE RADIO, Chao
et al. [7] argue that the method used to elicit preferences from users should take into
account the way in which these preference specifications will subsequently be used
1I-SPY is not a group recommender system in the most commonly assumed sense, because its
recommendations are made use of by individuals, not by members of a collaborating group.
But as we will see, the system addresses some of the same issues as systems that make recom-
mendations to groups.
20 Recommendation to Groups 601
for the generation of recommendations. Specifically, they argue that for a system that
chooses music to be played to a group, it makes more sense to elicit negative preferences
(e.g., expressions of dissatisfaction with particular music tracks) than to elicit more
detailed types of rating such as the ones mentioned in the previous subsection. If the
procedure that will be used for generating recommendations is designed mainly to avoid
the playing of music that is disliked by any member (cf. Section 20.3.2), effort expended
by one group member in discriminating among different degrees of liking may in fact
be wasted, since many of the songs that that member likes may in effect be vetoed by
another member anyway. An informal evaluation of ADAPTIVE RADIO suggested that
the focus on negative preferences was appropriate in the particular application setting
Sharing Information About Specified Preferences. In a recommender system for in-
dividuals, there is in general no person besides the user who has an immediate interest
in seeing explicitly specified preferences with a view to improving the current recom-
mendation process. In a group recommender, each member may have some interest in
knowing the other members’ preferences, for several possible reasons:
1. Saving of effort. Specifying preferences is usually seen by users as a tedious pro-
cess. (The avoidance of tedium is claimed by Chao et al. [7], as a supplementary
advantage of their focus on negative preferences.) If a group member m1knows
that another member m2with generally similar preferences has already specified
their preferences, m1may be able to save time and effort by copying at least some
of m2’s entries and then perhaps making some changes—especially if the system
makes it easy to do such copying and postediting.
2. Learning from other members. Another member’s preferences may be based in part
on knowledge or experience (e.g., concerning a particular vacation destination) that
the current member lacks.
An attempt to exploit both of these potential benefits is found in the TRAVEL DECISION
FORUM: A simple extension of a typical rating-scale dialog box allows the current
member optionally to view (and perhaps copy) the preferences already specified by
other members (see Figure 20.1). An additional feature that makes sense mainly if other
persons will be viewing the specifications is the option to add brief verbal explanations
or arguments for specific ratings.2Arguments can have various forms and functions
in group decision contexts (cf., e.g., Jennings et al. [21]). In a group recommendation
context, two typical functions are (a) to persuade other members to specify a similar
preference, perhaps by giving them information that they previously lacked; and (b) to
explain and justify a member’s preference even if the argument is not generalizable to
other members (e.g., “I can’t go hiking, because of an injury”).
Experience with this method of collaborative preference specification has revealed
further benefits beyond the two already mentioned:
2These arguments can be entered and viewed in pop-up windows that are not visible in Fig-
ure 20.1.
602 A. Jameson and B. Smyth
Fig. 20.1. Dialog box for the collaborative specification of preferences in the TRAVEL DECISION
FORUM. (The currently active group member is Claudia, the other two are Ritchie and Tina. The
preferences of each member are represented by the first letter of his or her name. Each scale
refers to a single attribute and ranges from −− for “Don’t want it” to ++ for “Want it”. The
highlighting of one cell for each attribute is added only when a compromise proposal has been
suggested, as explained in Section 20.3.1. Figure 1 of Jameson [18], reproduced with permission.)
1. Taking into account attitudes and anticipated behavior of other members. Some-
times the preference of the current member depends in part on the preferences
and/or the anticipated behavior of one or more other members. For example, if m1
sees that m2has specified a strong preference for tennis facilities, m1may want
to specify a similar preference, reasoning that if a hotel is found that offers tennis,
m1and m2will be able to play together. Otherwise, m1may genuinely not want to
emphasize tennis facilities, on the grounds that she would probably have no one to
play with anyway.
2. Encouraging assimilation to facilitate the reaching of agreement. A different rea-
son why m1may assimilate her preferences to those of m2is simply a desire to
minimize conflicts that may make it more difficult for the group to find a solution.
This pattern is especially likely in cases where m1was originally more or less in-
different between two possible preference specifications, before seeing that m2has
chosen the other one of them. The difference between this case and the previous
one is that here, m1’s true preference has not changed, but she has strategically
changed her specification of it.
In a similar vein, the more recently developed vacation recommender system CATS
(McCarthy et al. [28]; McCarthy et al. [29]) allows group members to achieve some
awareness of each other’s activities as they explore vacation options, working simulta-
neously around a DIAMONDTOUCH table, an environment that facilitates synchronous
work of group members on a common project (cf. Dietz and Leigh [12]). In the
20 Recommendation to Groups 603
Fig. 20.2. Illustration of several ways in which the CATS system enhances mutual awareness
among group members as they plan a skiing vacation. (Explanation in text. Figure 1 of McCarthy
et al. [28], reproduced with permission.)
overview in Figure 20.2, several examples can be seen: Each mountain icon in the large
map (a) represents one of the resorts that is being considered, the size of the icon re-
flecting the currently estimated overall preference of the group for that particular resort.
In the description of a particular hotel (b) the check mark or question mark next to the
color-coded icon for each group member indicates that group member’s estimated in-
terest in the hotel. The color-coded snowflakes on the map (a) indicate what resort each
member is investigating at the moment. Finally, each member can send a “critique” that
he or she is working on to the other members, thereby sharing his or her thoughts about
a particular option.
Although I-SPY’s preference specification is largely implicit, there are some phe-
nomena involved in the use of I-SPY that are similar to those that arise with collabo-
rative preference specification of the type we have seen with the TRAVEL DECISION
FORUM. These are related to the fact that each user sees the effects of the choices made
by other users, even if he does not recognize these effects as such. I-SPY exposes the
learned preferences of its community to searchers, in part by highlighting promoted
results in a search result list (see Figure 20.3 for examples and Smyth et al. [36], and
Smyth et al. [37] for further details). Thus, just as in the TRAVEL DECISION FORUM
a user can intentionally copy the preferences specified by another group member, in
I-SPY, the choices (and thus the implicit preference specifications) of each community
member will tend to be affected by the choices of previous searchers. In recent versions
of I-SPY, the current user can even see which individual users were responsible for the
promotion of a particular link (see Section 20.4.2). The purpose of the provision of this
type of information is to provide a sort of explanation of the recommendation of a given
604 A. Jameson and B. Smyth
Fig. 20.3. Screen shot of I-SPY illustrating several ways in which the current user is helped with
information derived from the behavior of other community members. (The “eyes” icon indicates
that a result has been promoted to a higher position in the result list than it would have had
link; this function will be discussed in more detail in Section 20.4; but this property is
relevant here in that it can lead to (a) a user thinking twice about whether to choose a
particular link because of knowing that others will see that he or she chose that link;
and (b) the tendency of users to be influenced by the fact that particular other users have
already expressed a degree of interest in a given link.
The idea of collaborative preference specification could be extended to some sys-
tems that do not yet to make use of it. For example, suppose that m1and m2jointly
make use of MOVIELENS’s buddy feature, which is the more recent implementation of
the ideas introduced with POLYLENS (cf. It is likely that
the movie tastes of m1and m2overlap to a greater extent than the interests of an arbi-
trary pair of movie-goers. How might m1benefit from being able to use m2s ratings
as a starting point for her own ratings? The most obvious case would be the one in
which many of m1’s ratings coincide exactly with those of m2; but even simply know-
ing which movies m2has rated at all might be helpful: Perhaps the most tedious thing
about using MOV IELENS is the job of finding movies that you have seen and so are able
to rate—a process that may require scrolling through a list of movies that extends over
many web pages. A list containing the set of movies that m2has rated but m1has not
rated might contain a higher proportion of movies that m1will be able to rate.
20 Recommendation to Groups 605
20.3 Generating Recommendations
No matter how a group recommender acquires the members’ preferences, the recom-
mendation for the group will in general be based on information about the preferences
of the individual group members. Therefore, some type of aggregation method is re-
quired, by which information about individual preferences is combined in such a way
that the system can assess the suitability of particular items for a group as a whole.3The
need to choose an aggregation method is the most obvious and intensively studied dif-
ference between group recommendation and recommendation for individuals. The topic
of preference aggregation is a multifaceted and complex one that has been addressed in
various scientific fields (see, e.g., Arrow [2], for a seminal contribution and Masthoff
[24] for a summary of this literature from the perspective of group recommendation).
The ideas to be discussed in this section overlap to some extent with ideas from these
other fields, but a number of new elements are introduced by the technical and practical
context of interactive adaptive systems. For example, much of the literature on pref-
erence aggregation considers large communities, whose members mostly do not know
each other (e.g., citizens of a country who are voting in an election); and the practical
contexts require aggregation methods that are technically simpler than many of those
that are feasible in adaptive web-based systems.
In this section, we will first discuss ways in which the aggregation problem has been
handled in group recommender systems to date. We will then discuss various compli-
cations that have not yet received as much attention as they will ultimately require.
We assume for now that the system is supposed to make recommendations concern-
ing just one decision (e.g., one film that is to be watched by the group). The case where
a sequence of decisions is to be made (e.g., concerning several TV programs that will
be watched on a given evening) will be considered in Section 20.3.3.
20.3.1 Approaches to Preference Aggregation
Although the various approaches differ in the ways in which they gather and represent
users’ preferences, almost all approaches make use of one of three schemas: (a) merging
of sets of recommendations, (b) aggregation of individuals’ ratings for particular items,
or (c) construction of group preference models.4
Merging of Recommendations Made for Individuals. In cases where what is to be
presented is a set of candidate solutions, among which the group is to select one for
3It is not actually logically necessary for group recommendations to be based on information
about the preferences of individual members. For example, a group movie recommender might
somehow acquire the knowledge that Walt Disney movies tend to be suitable when parents are
taking their small children to a movie, even if the recommender system has no information
about how parents and children, respectively, tend to evaluate Walt Disney movies. But since in
practice almost all group recommenders start with information about preferences of individual
members, we will view the problem of making recommendations for a group as involving
preference aggregation.
4Somewhat similar distinctions among aggregation approaches have been made by, among oth-
ers, O’Connor et al. [30]; Kay and Niu [22]; and Yu et al. [39].
606 A. Jameson and B. Smyth
Fig. 20.4. Example of a display of group recommendations in POLYLENS. (Adapted with per-
mission from an image supplied by John Riedl. Explanation in text.)
adoption, a simple aggregation method is that of generating a small number of recom-
mended solutions for each member and then merging them into a single list:
1. For each member mj:
For each candidate ci, predict the rating rij of ciby mj.
Select the set of candidates Cjwith the highest predicted ratings rij for mj.
2. Recommend jCj, the union of the set of candidates with the highest predicted
ratings for each member.
This method was one of those considered for the POLYLENS system (O’Connor et
al. [30]). Because of the simple relationship to the generation of recommendations for
individuals, this method can be implemented easily as an extension of a recommender
for individuals, making use, for example, of any explanation facilities of the individual
recommender. In particular, if each recommendation is accompanied by a display of
the (predicted) ratings of each member, the members may have quite a good basis for
choosing a truly acceptable solution. On the other hand, this approach does presuppose
that the members will play an important role in the final decision making, since the list
of recommendations does not in itself indicate which solutions are best for the group
as a whole. In fact, in the worst case each proposed solution might be excellent for one
member but terrible for all of the others. More generally, this method ignores the set
of solutions that are not expected to be especially appealing to any member but which
might represent the best solution for the group as a whole.
Since this method is rarely even considered for use in group recommenders, we will
not discuss it further.5
Aggregation of Ratings for Individuals. This commonly applied approach starts with
the assumption that, for each candidate item ciand each member mj, the system can
predict how mjwould evaluate (or rate)cjif he or she were familiar with it:
1. For each candidate ci:
For each member mjpredict the rating rij of ciby mj.
Compute an aggregate rating Rifrom the set {rij}.
2. Recommend the set of candidates with the highest predicted ratings Ri.
5A closely related method is considered by Yu et al. [39] for the recommendation of sequences
of TV programs; essentially the same drawbacks apply in that context as well.
20 Recommendation to Groups 607
This approach is illustrated by POLYLENS, as can be seen in Figure 20.4. The three
right-hand columns in the screen shot display ratings that have been predicted for
individual users via the same collaborative filtering method that MOVIELENS uses
for individual users. The column labeled “GROUP” shows the aggregated rating. In
POLYLENS, the aggregation method is very simple:
Ri= min
jrij .(20.1)
That is, instead of looking for the movie with the highest average rating, POLYLENS
applies the strategy of “least misery”: It bases its recommendation on the lowest pre-
dicted rating for each candidate, preferring candidates for which the lowest predicted
rating for any group member is relatively high. Other plausible aggregation methods
will be mentioned in Section 20.3.2.
Construction of Group Preference Models. The second widely applied approach to
aggregation does not involve any predictions of ratings of individual users. Instead,
the system somehow uses its information about the preferences of individual group
members to arrive at a model of the preferences of the group as a whole:
1. Construct a preference model Mthat represents the preferences of the group as a
2. For each candidate ci,useMto predict the rating Rifor the group as a whole.
3. Recommend the set of candidates with the highest predicted ratings Ri.
With regard to Step 1: There are even more possible methods for the construction of
group preference models than for the aggregation of individual ratings, since group
preference models can take many different forms.
In some cases, the group preference model can be seen as an aggregation of individ-
ual preference models. An example is given by LET’s BROWSE: Each individual user’s
profile is a set of keyword/weight pairs that reflects the typical content of the pages
that this user likes to view. The system computes a model of the group by forming a
linear combination of these individual models. From then on, it no longer has to consult
the individual models when making recommendations. Similarly, in the context of an
in-vehicle multimedia recommender (Yu et al. [40]) and a TV program recommender
(Yu et al. [39]), Yu and colleagues introduced and evaluated a method for constructing
a group preference model on the basis of individual preference models, using a notion
of distance between preference models.
In other cases, a group preference model may represent an aggregate of preference
models for subgroups, rather than for individual members. This approach is exemplified
by the system INTRIGUE (Ardissono et al. [1]), which is designed to help tour guides
who need to design tours for heterogeneous groups of tourists that include relatively ho-
mogeneous subgroups (e.g., “children”). The tourist group leader divides the tour group
into several categories of homogeneous users and specifies a preference model for each
such subgroup. The group model is then a weighted average of the subgroup models,
with the weights reflecting the importance of the subgroups (e.g., the subgroup of dis-
abled persons is considered especially important because of the special requirements of
its members).
608 A. Jameson and B. Smyth
The TRAVEL DECISION FORUM takes the focus on group models one step further:
In fact, the main function of the system is to help the group members, for each aspect
of the vacation that the group members are planning, to arrive at a group preference
model that all members have agreed to—that is, at a way of filling out each preference
specification form (such as the one shown in Figure 20.1) in such a way that it reflects
the preferences of the group as a whole. If we look at the system in this way, the system
can be seen as one that recommends specific preferences for the group model (e.g., a
rating of ++ for the attribute “Sauna” in Figure 20.1).
I-SPY creates a group (or community) preference model directly on the basis of data
concerning the behavior of individual group members, bypassing the level of individ-
ual preference models—partly because of privacy considerations, as will be discussed
shortly. I-SPY’s basic community preference model consists of a record of queries that
have been submitted (by the community of searchers) and the result pages that have
been selected for these queries, along with frequency information for these selections.
When deciding to what extent to promote a particular search result for a particular
community, I-SPY bases its decision on an estimate of how relevant this result page is
likely to be for the current query. This estimate is based on the frequency with which
this page has been previously selected by community members for the current query
and for similar queries.
Choosing Between Rating Aggregation and Group Preference Models. Construct-
ing a preference model for the group has the clearest advantages when the group mem-
bers will have an opportunity to examine and/or negotiate about the group’s model
before or after it is actually applied. In this case, for example, the users of INTRIGUE
or the TRAVEL DECISION FORUM could settle among themselves once and for all the
relative priorities of historical interest and entertainment, instead of debating this issue
with respect to each individual attraction. This type of process will be discussed further
in Sections 20.4 and 20.5.
If, on the other hand, the group model will be created and applied in the background,
without inspection by the group members, the question of whether a group model is
better is a more technical one that involves considerations such as efficiency and the
quality of recommendations. For example, O’Connor et al. [30] discuss various ways
in which POLYLENS could have been designed to create a model of each group (e.g.,
a “pseudo-user” who represents the interests of the group as a whole) before any rec-
ommendations were generated—and some typical consequences of such group models.
For instance, a group model might (accurately or not) recommend a movie for which
the predicted rating of each individual member was low—something that cannot happen
with recommendation-level aggregation.
Another advantage of a group preference model concerns its potential privacy bene-
fits. Recording and maintaining individual user profiles will typically raise privacy con-
cerns, especially if these profiles are owned by some third-party system on the server
side. In contrast, the use of a group preference model may go a considerable way toward
alleviating these privacy concerns. I-SPY is a case in point. Our web search behavior
can be surprisingly revealing when it comes to understanding the likes and dislikes of
an individual—far more revealing and valuable to an eavesdropper than movie or mu-
sic preferences, for example. I-SPY’s use of a community-based profile, in which the
20 Recommendation to Groups 609
search behavior of individual searchers is merged, means that the search preferences of
any individual searcher can no longer be reconstructed.
20.3.2 Alternative Goals and Procedures for Aggregation
Even once a general approach has been chosen, the question arises of what particular
computational procedure (or mechanism) should be used for the aggregation. This is
the single question in this area that has received the most attention. The problem is
that there are a number of goals that may be desirable in any given situation (e.g., total
satisfaction, fairness, and comprehensibility), and conflicts between them can easily
arise. In this section, we give several examples of such goals.
Whereas many treatments of these issues (see, e.g., Masthoff [24]; Yu et al. [39]) de-
vote considerable attention to mathematical formulas, quantitative examples, and tech-
nical concepts, we will focus on the basic underlying issues and concepts and how they
relate to realistic application scenarios.
Maximizing Average Satisfaction. Suppose at first that we are taking the approach
of aggregating individual ratings. In this case, the goal of maximizing average satisfac-
tion can be achieved by an aggregation function that computes some sort of average of
the predicted satisfaction of each member for use as a basis for the selection of can-
didates (see Equation 20.2). The POCKET RESTAURANTFINDER (McCarthy [26]; cf.
Section 20.2.1) applies a variant of this formula to the predicted ratings of restaurants by
members of a group who are preparing to go out to dine together. The G.A.I.N. system
of Pizzutilo et al. [32], which presents news items on a wall display or an information
kiosk, uses a more complex variant of this formula that takes into account uncertainty
about which users will be viewing the display at any given time; a similar procedure
is applied in FIT (Goren-Bar and Glinansky [14]), which recommends TV shows for
Ri=average({rij})=1/n ·
rij .(20.2)
If the predicted ratings are not thought to represent satisfaction accurately, some trans-
formation of them can be used, such as the square of the rating; some results concerning
transformations of this sort are given by Masthoff [24].
Minimizing Misery. Even if the average satisfaction is high, a solution that leaves one
or more members very dissatisfied is likely to be considered undesirable. Even the most
ego-centered group member may not want to have to interact with another member who
is thoroughly dissatisfied; and such a member may refuse to go along with the solution
in any case. In POLYLENS, the minimization of misery is the only criterion applied (see
Equation 20.1 above). It is also possible to take this factor into account as a constraint
that must be fulfilled by a solution: The lowest predicted rating must not fall below a
given threshold.
610 A. Jameson and B. Smyth
Ensuring Some Degree of Fairness For similar reasons, a solution that satisfies ev-
eryone just about equally well is in general preferred to one that satisfies some at the
expense of others—all other things being equal. Even more than in the case of minimiz-
ing misery, the goal of ensuring fairness is in general combined with some other goal.
After all, no-one wants a perfectly fair solution that makes everyone equally miserable.
For example (again assuming the approach of aggregating individuals’ ratings), the ag-
gregation of predicted individual ratings might include a penalty term that reflects the
amount of variation among the predicted ratings, as in Equation 20.3:
Ri=average({rij})w·standard-deviation({rij }),(20.3)
where wis a weight that reflects the relative importance of fairness.
Treating Group Members Differently Where Appropriate. In some situations, it
is generally agreed that the preferences of some group members need to be treated
differently than those of others. If two hosts are planning a visit to a restaurant with a
visitor from out of town, they are likely to give high priority to the visitor’s preferences,
requiring only that the solution is not entirely unsatisfactory for themselves (cf. e.g.,
Kay and Niu [22]). In INTRIGUE, the tourist guide is able to assign higher weights to
subgroups such as those of disabled persons or children, on the assumption that these
group members are less able to put up with solutions that are even partly unsatisfactory
for them.
Discouraging Manipulation of the Recommendation Mechanism. The problem of
manipulation is illustrated by experience with an early version of the system MUSICFX
(McCarthy and Anagnost [27]), one of the earliest group recommender systems, which
automatically selected music genres for the music to be played in a fitness studio:
Although the system essentially applied an averaging procedure to construct a group
model from individual preference models, the system also enforced a constraint of the
type mentioned above in connection with the “least misery” criterion: Any music genre
that was “hated” by any member currently in the gym was removed from the list of pos-
sible genres to play. Some users were observed to force an immediate change of genre
by adapting their specifications to indicate that they “hated” the genre currently being
played—even if they really didn’t mind it but simply liked it less well than some other
The potential for manipulation is even more obvious in the TRAVEL DECISION FO-
RUM, in which one group member can often see the preferences specified by the other
members. For example, suppose that in Figure 20.1 Claudia’s true preference regarding
the presence of a sauna was (“Don’t care”): Instead of selecting the middle box in the
scale, she might be inclined to select the left-most box (indicating strong disapproval
of the availability of a sauna), so as to compensate for the positive preferences speci-
fied by the other group members Ritchie and Tina, expecting that the aggregated group
preference for a sauna will end up being closer to her own.
When this type of insincere specification of preferences occurs, the aggregation
algorithm used will be operating on false premises, since the algorithms presuppose
that a group member’s expressed preferences reflect his or her true preferences.
20 Recommendation to Groups 611
One way of making manipulation difficult is to make it impossible for users to see
each others’ preferences before specifying their own: If you don’t know what the other
members prefer, it is hard to distort the resulting recommendation in your own direction
by specifying an insincere preference. But users may be able to guess other members’
preferences (at least roughly); and in any case, as we have seen, there are advantages to
allowing members to see each others’ preferences at an early stage.
Manipulation is most likely to be possible if the input that the system uses for mak-
ing its predictions consists of explicit preference specifications; with implicit inference
of preferences, users are much less likely to be able to see how they could influence a
recommendation by acting in some particular way. But exceptions can occur; for ex-
ample, in I-SPY, user can quickly notice that, when they choose a particular link for a
given query, that link gets promoted in the search result list for that query. It is then an
obvious next move to click on links that one would like to promote (e.g., pages writ-
ten by the user), regardless of their actual relevance to the query. One can view this
type of manipulation as an alternative form of search engine spam, because subsequent
users will see potentially irrelevant results being unjustifiably promoted to positions of
prominence. As a potential solution to this problem, Briggs and Smyth [3] propose the
use of an explicit model of trust that provides a filtering mechanism with a view to
eliminating the contributions of these manipulative selections: The selections of indi-
vidual users are evaluated for their reliability. In the simplest sense, a result selection is
considered to be reliable if the same link is subsequently reselected by a certain min-
imum number of searchers for similar queries in the future. This information is used
for (among other things) the evaluation of the trustworthiness of individual users, so
that recommendations that stem from the activities of users with low trust values can
be eliminated or demoted. Preliminary evaluation results suggest that the technique is
capable of improving recommendation accuracy.
A different approach to discouraging manipulation is to have the system use an
aggregation method that is inherently nonmanipulable: It is never in the interest of
a given user to specify any preference other than the one that he or she really has.
To return to the example with the TRAVEL DECISION FORUM given above: A simple
nonmanipulable aggregation mechanism uses as a preference for the group as a whole
concerning a given attribute the median of the individual preferences for that attribute
(i.e., the one that falls exactly in the middle of an ordered list of all preferences). In our
example, Claudia will not be able to drag down the group preference for a sauna below
+by specifying a low preference herself, since the median preference will be +for any
preference that she specifies between −− and +. (It will be left as an exercise for the
reader to verify that, with the use of the median mechanism in this setting, no group
member could ever benefit by specifying a preference insincerely.)
In general, many nonmanipulable mechanisms may exist for any given preference
aggregation problem. In the research area of automated mechanism design (see, e.g.,
Conitzer and Sandholm [8]; Conitzer and Sandholm [9]; Sandholm [33]), methods are
developed for automatically generating aggregation methods for a particular setting that
(a) satisfy the constraint of being nonmanipulable (at least in that particular setting) and
(b) also respect other constraints as well (e.g., maximizing average satisfaction and/or
ensuring a certain degree of fairness). The methods introduced by Conitzer and Sand-
612 A. Jameson and B. Smyth
Fig. 20.5. Part of a dialog box in the TRAVEL DECISION FORUM via which an aggregation proce-
dure can be selected. (The slider, which is active only when a mechanism is to be generated auto-
matically, determines the relative weight of average satisfaction and fairness—cf. Equation 20.3.
Adapted with permission from Figure 2 of Jameson [18].)
holm were implemented in the TRAVEL DECISION FORUM, along with the median
mechanism just mentioned and two other mechanisms. The system administrator or
an individual group member can request that an automatically designed mechanism be
used for the generation of recommendations and can specify the properties that this
mechanism should have (see Figure 20.5); the system then generates on the fly a mech-
anism that fulfills the specified constraints. The main issues that arose concerning the
appropriateness of such automatically designed mechanisms were whether they were
sufficiently comprehensible (or “transparent”, in the terms of Figure 20.5) and accept-
able to users. These issues will be discussed in the next subsection.
Ensuring Comprehensibility and Acceptability. As will be discussed in Section 20.4,
group members sometimes like to be able to understand the rationale behind a recom-
mendation. In particular, they may want to check to what extent acceptability criteria
such as the ones discussed earlier in this section are being fulfilled. Even with ingenious
visualizations such as those that will be shown in Section 20.4, it may be difficult for
a system to explain a recommendation if the mechanism by which the recommenda-
tion was generated is inherently complex and/or counterintuitive. Therefore, it may be
worthwhile to choose an inherently comprehensible mechanism even if it is not the best
mechanism in terms of the other criteria.
An example of a comparison of mechanisms in terms of their inherent compre-
hensibility and acceptability is the exploration of automatically designed nonmanipu-
lable mechanisms in the context of the TRAVEL DECISION FORUM (cf. the previous
subsection and Jameson et al. [20]). One fundamental limitation of the automatically
designed mechanisms is the fact that such a mechanism cannot be represented with a
simple formula (such as the formula for the average or the median) but rather has to
be represented by a table that specifies, for each possible combination of preferences
of individual users, which item should be chosen.6Therefore, a group member can-
not apply the mechanism mentally in order to predict or understand recommendations.
6Actually, automatically generated mechanisms are often nondeterministic: For each possible
combination of preferences, the mechanism specifies a vector of probabilities associated with
20 Recommendation to Groups 613
Moreover, unless special acceptability constraints are applied in the mechanism gener-
ation process, the generated mechanism may give rise to recommendations that strike
people as counterintuitive and inappropriate (e.g, proposing for some combinations of
individual preferences an outcome that none of the group members likes, even though
there exist outcomes that some members like). In sum, automated mechanism design
is an approach that deserves further attention, but special attention must be paid to the
goal of ensuring adequate comprehensibility and acceptability.
20.3.3 Further Complications Concerning Preference Aggregation
The often conflicting goals discussed in the previous subsection are in themselves
enough to make the problem of choosing a suitable aggregation procedure a difficult
one. But there are additional complications that need to be taken into account in some
Generating Recommendations Concerning Multiple Decisions. So far, we have
been focusing on the situation in which a recommender will make recommendations
to a group concerning just one decision. But often the group members will expect a
system to make recommendations concerning a larger set of decisions, either at the
same time or in succession: INTRIGUE’s tour guide will choose several sights to visit;
the music selection systems will choose one song after the other; and a TV program
recommender will recommend several programs for a group to watch in succession.
In this type of situation, the procedure for generating recommendations about the
entire sequence can be related to a procedure with respect to individual decisions in
any of several ways. Figure 20.3 compares three approaches, which differ in terms of
several criteria.
1. The simplest approach is to treat each decision separately, ignoring the fact that
there will be a sequence of decisions. Because of its simplicity, this approach tends to be
computationally simple and easy to explain to users. One drawback is that a goal such
as ensuring fairness can be taken into account only with respect to individual decisions,
whereas it can be advantageous to consider it with regard to the entire set of decisions.
For example, it may seem fair enough to recommend a single TV program that is much
less attractive for one group member than for the others as long as that group member’s
overall satisfaction with the sequence of programs is comparable to that of the other
members. Trying to ensure approximately equal satisfaction among a group members
with each individual program in the sequence may rule out too many options that would
be attractive for the group as a whole. An even clearer limitation of this approach arises
in cases where the system can acquire feedback about the results of each decision be-
fore making recommendations concerning the next one. Suppose, for example, that one
group member was especially dissatisfied with a given TV program, even though this
dissatisfaction was not predicted in advance. It may be feasible and desirable to bias the
recommendations concerning one or more subsequent programs in favor of that group
member, so that he or she a ultimately reaches an appropriate level of satisfaction; but
the various possible outcomes, which is to be used for the random selection of one of the
614 A. Jameson and B. Smyth
Table 20.3. Positive (+), negative (), and intermediate (+/) aspects of three approaches to
the treatment of a sequence of decisions by a group recommender.
Approach to treating a sequence of decisions
Criterion Independently As one complex
decision Individually but with
consideration of other
complexity + − +/−
Comprehensibility + − +/−
Appropriateness of
evaluation criteria − + +/−
Ability to take into
account actual results
of individual
− − +
Applicability when
decisions and
members involved are
not known in advance
+ − +/−
Ability to take into
account additional
ordering constrains
− + +/−
this type of compensation is not possible if the decisions are treated completely in-
dependently. Similarly, this approach cannot deal with other constraints that concern
relationships among members of the sequence (e.g., the possible undesirability of pre-
senting two very similar items in succession, cf. the discussion of FLY TR AP below; or a
possible tendency of earlier items in the sequence to have a generally larger impact on
users than later items, cf. Masthoff [25]).
2. The opposite approach is to view the recommendations for the entire sequence
as a single recommendation problem, much like that of recommending complex items
(such as vacations) that differ with respect to a number of attributes. This approach is
applicable only if it is known in advance what particular sequence of decisions is going
to be made and what group members are going to be involved (e.g., how many times
and on which particular occasions a group of diners is going to go out to dine together).
This approach makes it straightforward to apply criteria such as fairness to entire se-
quences. On the other hand, it does not make it possible to take into account the results
of previous decisions, since the decision making process for the entire sequence is com-
pleted before any decisions are executed. Also, the necessary computational procedures
tend to be more complex; for example, the number of possible sequences may be too
large for it to be feasible for the system to iterate through all possible sequences and
evaluate their suitability.
3. An approach that lies between these two extremes starts with the idea of treating
each decision problem separately but makes some adjustments to take into account the
20 Recommendation to Groups 615
fact that a sequence is being dealt with. For example, each individual decision might be
approached with the goal of maximizing overall satisfaction; but if the system notices
that, up to a given point in time, a given member has been less satisfied than the others,
his or her satisfaction can be given greater weight in subsequent decisions, until the
discrepancy has been eliminated. This approach is able to take into account the actual
results of decisions (as opposed to only the predicted results). Although more complex
than independent treatment of decisions, the method may be reasonably explainable if
it corresponds with familiar decision making schemas from everyday life (e.g., “That
last program was awful for Mary, so let’s give her a break this time.”) This approach
does not require the set of decisions to be known in advance, but it can be applied most
effectively if a good deal is known. For example, if the system has decided to grant
a certain amount of extra satisfaction to a particular group member while making the
subsequent recommendations, it will be helpful to know how many decisions remain to
be made.
An example of this third approach is found in the FLY TRAP music selection system
(cf. Section 20.2.1): The system has to choose songs one at a time, because the set of
persons who are present to hear them frequently changes. But its selection procedure
does take into account constraints imposed by a “DJ agent” that tries, for example, to
avoid abrupt and distracting changes of genre.
20.3.4 Preference Specifications That Reflect More Than Personal Taste
In most analyses, it is assumed that the preferences specified by a group member repre-
sent simply the desires of that individual member (e.g., in a TV context, the programs
that the group member would watch if he or she were watching alone). But in some set-
tings, a group member may be taking into account the assumed interests of other mem-
bers when expressing his or her own preferences. For example, we noted in connection
with the TRAVEL DECISION FORUM that users sometimes expressed preferences in
such a way as to minimize the likelihood of conflict. To take a more extreme example,
consider a mother who is taking her two children to the movies and whose primary
motivation is to find a movie that the children will like and that she herself will not
hate having to sit through. In this case, her preference specification (e.g, a high rating
for a particular children’s movie) will reflect only to a minimal extent her own taste in
movies. So it would not be appropriate, for example, for a recommender system to look
straightforwardly for a “compromise” between the mother’s expressed preference and
the preferences expressed by the children (though some more sophisticated aggregation
of the various expressed preferences might well make sense).
An additional complication arises when the group members’ preferences reflect not
just subjective tastes but also knowledge that may be relevant to the choices of the other
members as well. For example, suppose that a member m1of a travel group who is
especially familiar with Switzerland expresses a strong preference for a given Swiss ski
resort: The other group members may be willing to give extra weight to m1s opinion,
even if their own evaluation criteria for resorts are somewhat different, on the grounds
that the resort in question is especially likely to be good at least according to m1’s
616 A. Jameson and B. Smyth
criteria, which overlap to some extent with the criteria of the other group members.7An
ad hoc way of taking differences in knowledge into account is to assign greater weight
to the preferences of more knowledgeable group members; but this method does not
address in a principled way the relationship between knowledge and preferences.
20.3.5 At What Points Can These Complexities Be Dealt With?
After this lengthy discussion of conflicting goals for preference aggregation procedures
and additional complexities, the reader may be wondering whether it will ever be pos-
sible to deal with these issues adequately in group recommender system. Fortunately,
the issues can be dealt with by different people at different points in time:
1. The system’s designers and/or deployers may specify an appropriate means of han-
dling each problem: The persons who are designing a group recommender—or
arranging the deployment of an existing recommender in a given context—can
consider each of the issues discussed above with regard to their particular target
group and application setting and work out some locally appropriate solution. For
example, the designers of POLYLENS thought that the “least misery” aggregation
function would be appropriate because they expected most groups of people who
go to see a movie together to be small (i.e., 2 or 3 members); for settings involving
larger groups, the same function would probably lead to too many cases in which a
solution that would be liked by many members would in effect be vetoed by the one
person who liked it least. If the designers of a restaurant recommender anticipate
that there will often be some group members who are familiar with the restaurants
in question and others who are not, they might look for a principled way of treating
the two types of group member differently.
2. The system’s users may select a suitable preference aggregation method for each
decision: A system can allow the users to decide what aggregation mechanism
is to be used, either before any recommendations are made or during an iterative
process of requesting recommendations and adjusting the aggregation function. For
example, with INTRIGUE the tour guide can specify a different set of subgroup
weights for each tour group. As was mentioned above, a variety of aggregation
mechanisms can be chosen in the TRAVEL DECISION FORUM. This idea could be
adopted in many recommendation settings.
3. Users can take any remaining factors into account when evaluating specific rec-
ommendations and negotiating about the final decision: If the system presents a
number of recommendations and allows users to choose which one(s) they want to
adopt, it may not be necessary for the system itself to deal with all of the subtle
problems that can arise. Instead, the users themselves may be able to take these is-
sues into consideration, making use of their long experience with social interaction
and relationships. After all, even the most subtle of the issues discussed in this sec-
tion concern matters that people are accustomed to dealing with in everyday life. If
7Hastie and Kameda [16] show how aggregation procedures can be compared in terms of their
suitability for arriving at accurate decisions even in settings where the group members are as-
sumed to have identical preferences—that is, where knowledge aggregation rather than pref-
erence aggregation is involved.
20 Recommendation to Groups 617
the group recommender is designed on the assumption that there are some aspects
of the decision problem that are better dealt with by the users themselves, the func-
tion that the system serves is mainly decision support rather than decision making.
In these cases, special importance should be assigned to the third and fourth sub-
tasks of a group recommender (explaining recommendations and supporting final
decision making), which will be discussed in the next two sections, respectively.
20.4 Explaining Recommendations
20.4.1 Motivation
Given the many ways in which recommendations for a group can be derived—and the
often conflicting goals that can be pursued—it is natural that group members should
want to understand to some extent how a recommendation was arrived at—and in par-
ticular, how attractive a recommended item is likely to be to each individual group
Recommender systems for individuals often accompany each recommendation with
some sort of analysis of its predicted acceptability; the analysis may range from a sim-
ple index of the system’s confidence to a complex visualization of the pros and cons
of the recommended solution (see, e.g., Herlocker et al. [17], and the chapter in this
volume by Schafer et al. [34]). With group recommenders, it is in principle possible
to present such an analysis for each individual member, for the group as a whole, and
perhaps for subsets of members. A member m1may be interested in the analysis for
m2because m1considers it important that m2be satisfied, because m1wants to make
sure that she is getting “as good a deal” as m2, or simply in order to understand how
the recommendation was derived.
20.4.2 Treatment in Existing Group Recommenders
As can be seen in Figure 20.6, LETSBROWSE explains each of its web page recom-
mendations by listing the keywords in the page that it assumes to be of interest to all
group members. By showing where these keywords are located in each member’s pro-
file, the system also allows each user to guess how interesting each member will find the
page. In the example in the figure, it looks as if the (hypothetical) group member George
Lucas will be less enthusiastic about the page than the member Bill Gates, given that Lu-
cas is only marginally interested in technology. As some of the systems to be discussed
below suggest, it might be a worthwhile further step for LETSBROWSE to present an
explicit estimate of the likely interestingness of the page for each group member and
perhaps for the group as a whole. In this way, for example, Lucas might more readily
accept the system’s recommendation of the page involved in Figure 20.6, seeing quickly
that it is more interesting to other members than it is to him; or, depending on his overall
attitude, he might object to the recommendation for just that reason. On the other hand,
since (as was mentioned in Section 20.3.1) LET’s B ROWSE uses a group-level model
to compute its recommendations, the computation of predictions for individual group
618 A. Jameson and B. Smyth
Fig. 20.6. Screen shot of LET’s B ROWSE that shows how the system explains a recommendation
for three (hypothetical) users of a particular web page. (Adapted with permission from an image
supplied by Henry Lieberman.)
members for presentational purposes would involve additional overhead, and it would
not reflect the way in which the system actually arrives at its recommendations.
POLYLENS gives an idea of how a display that shows predictions for individuals
might look. Since POLYLENS uses collaborative filtering, it cannot explain a movie rec-
ommendation in terms of the movie’s content; but it does show the predicted rating for
each group member and for the group as a whole (see Figure 20.4 above). In addition to
explaining each recommendation in terms of the underlying predictions for individuals,
this visualization makes it possible for the attentive user to notice how group recommen-
dations are generated—via the “least misery” principle (Equation 20.1)—by comparing
predictions for the individual members with the predictions for the group. Incidentally,
more than 90% of the users surveyed stated that they had no privacy concerns about
having their predicted ratings shown to other group members—a result that encourages
the development of additional methods that expose individual-level predictions to all
group members.
INTRIGUE offers a type of explanation (Figure 20.7) that is partly similar to that of
POLYLENS: It shows the predicted attractiveness of each recommended attraction for
the tourist group as a whole; but instead of simply presenting a predicted attractiveness
for each subgroup, it explains verbally the aspects of the attraction that are likely to
20 Recommendation to Groups 619
Fig. 20.7. Example of INTRIGUE’s main explanation method. (Adapted with permission from
Figure 3 of Ardissono et al. [1].)
Fig. 20.8. Part of a visualization from the FLY TR AP music selection system. (The songs most
likely to be played are shown in the middle of the display. The color coding reflects the estimated
degree of interest of the two current listeners Kris and Andy, as well as the influence of the DJ
agent. Adapted with permission from Figure 1 of Crossen et al. [11].)
appeal to that subgroup (not mentioning the less appealing aspects). This type of expla-
nation seems helpful for the tour guide who would like all group members to accept the
recommendation, but it does not convey a clear idea of how attractive the recommended
tour is to each subgroup.8
8INTRIGUE also offers a different type of explanation that shows only predictions for individual
subgroups; see Figure 8 of Ardissono et al. [1].
620 A. Jameson and B. Smyth
FLYTRAP offers a completely different type of visualization (see Figure 20.8),
which indicates which users are present in the room in which music is being played,
what songs are more or less likely to be selected for playing, and how these songs are
expected to be evaluated by the users who are present and by the “DJ agent”. Since the
users of FLYTRA P do not have an opportunity to influence the choice of songs directly,
the purpose of this visualization is to convey a general understanding of how the system
I-SPY incorporates a number of strategies for making clear the reasons for the pro-
motion of a given link in a search result list. Since the users of I-SPY are not viewed as
working together when they look for relevant links, the focus here is not on showing the
desirability of an option for particular group members with a view to resolving conflicts.
Nonetheless, it has proven worthwhile to provide information about how other commu-
nity members have dealt with the page in question in the past, because such information
helps the current user to judge its value for him- or herself. Types of information offered
include (a) related queries for which the page in question has been selected as a promis-
ing result (see Figure 20.3); (b) quantitative and temporal information such as “10% of
searchers have also selected this result for similar queries as recently as 15 minutes
ago” (Coyle and Smyth [10]); and (c) the names of the users who are responsible for
the promotion of the page (a by-product of the antimanipulation measures discussed in
Section 20.3.2; see Briggs and Smyth [3]).
The TRAVEL DECISION FORUM introduces two novel, complementary methods
that aim to provide a more detailed picture of the consequences of a given proposal for
each group member:
1. The first method automatically follows from the use of the preference specifica-
tion form for the presentation of proposals (see Figure 20.1). Since both the specified
preferences and the recommended joint preferences are shown on the same set of scales,
the user can quickly see which group members should be most / least satisfied with a
given proposal (i.e., the ones whose preferences are closest to / farthest from the high-
lighted cells). Also, with a bit of practice the user can see more complex patterns (e.g.,
Claudia might notice that “Tina and Ritchie have generally similar preferences, and
they usually get their way, while my preferences have little influence”). Any verbal ar-
guments associated with the other members’ stored preferences add further detail to the
picture of how they would evaluate a given proposal.
2. The second method takes into account the fact that any graphical explanation of
a recommendation is likely to be less interesting, vivid, and memorable than the type
of feedback that group members get while they are interacting face to face: A member
who is disappointed with a proposal may complain about specific aspects of it in an
emotional manner, formulating (or repeating) arguments. In settings where all group
members are physically present in front of the group recommender system, this type
of face-to-face discussion is likely to occur spontaneously. For settings in which no
such direct communication is possible, the TRAVEL DECISION FORUM tries to recap-
ture some of the flavor of face-to-face interaction through animated characters: It is
assumed that at any given moment only one group member will be interacting with the
system; each of the other members is represented by an animated character who bears
that member’s name. Whenever the system (represented by an animated character called
20 Recommendation to Groups 621
Fig. 20.9. Use of animated characters in the TRAVEL DECISION FORUM to represent the likely
responses of the absent group members to a proposal. (Here, the representative of Tina is respond-
ing to the proposal shown in Figure 20.9. Adapted with permission from Figure 3 of Jameson
the mediator) has recommended a particular joint preference model for a given value
dimension (such as “health facilities”), he asks the representatives of the absent group
members to comment on it in turn. Parts of a typical performance of a representative
are shown in Figure 20.9.
This type of simulated reaction can heighten the group members’ awareness of the
other members’ points of view—including their motivational orientations—and over-
come the natural tendency to focus on one’s own evaluations. Also, like the explana-
tions of INTRIGUE, these presentations are selective, focusing on the most important
considerations for each group member. The user can switch attention back and forth
between the animated agents and the graphical explanation, because the two types of
explanation make use of largely complementary communication channels.
20.4.3 Concluding Remarks About Explanations
These examples from existing systems illustrate (a) that there are many types of infor-
mation that can be conveyed by an explanation of the system’s recommendations and (b)
that the function of such explanations is not necessarily to convince the group members
that the system’s recommendations should be accepted. Instead, the system’s explana-
tions are often best seen as information that puts the group members in a better position
to make a final decision, which may deviate radically from the recommendations made.
The process of arriving at a final consensus is the subject of the next section.
20.5 Helping Group Members to Achieve Consensus
Even with a recommender system for individual users, no matter how appropriate and
compelling the system’s recommendations and explanations may be, there is usually
622 A. Jameson and B. Smyth
no guarantee that any of the recommendations will be adopted. With individual recom-
menders, although the decision process may be complex, it typically takes place within
the mind of a single person. With a group recommender, extensive debate and negotia-
tion may be required, which may be especially problematic if the members are not able
to communicate easily.
20.5.1 Treatment in Existing Systems
Group recommender systems have tended not to provide explicit support for the process
of arriving at a final decision. Such support may in fact not be required if any of the
following conditions hold:
1. The system simply translates the most highly rated solution into action without
requesting the consent of any users.
This method is applied by the music selection systems ADAPTIVE RADIO,FLY-
TRAP, and MUSICFX, which select and play music autonomously on the basis of
the preferences of the group members who are present. It would in fact usually
be impractical to allow the persons who happen to be present in a public space to
debate about each piece of music that is to be played.
2. It is assumed that one group member is responsible for making the final decision.
LETSBROWSE is based largely on the assumption that one group member controls
the pointing device and will therefore make most of the decisions about what pages
to visit. With INTRIGUE, it appears to be assumed that the tourist guide will decide
what tour should be taken.
3. It is assumed that group members will arrive at the final decision through conven-
tional discussion (e.g., face to face or by phone).
An especially clear example where this assumption is justified is the situation where
several group members are working simultaneously with the CATS vacation rec-
ommender system (cf. Section 20.2.2) on the DIAMONDTOUCH interactive table-
top. In addition to the usual broad bandwidth of face-to-face communication, the
group members can refer during their negotiations to the various information dis-
plays provided by the system.
The only other system we know of that specifically supports communication among
group members for the purpose of final decision making is the TRAVEL DECISION
FORUM—understandably, since this system is designed specifically for groups of users
who usually cannot communicate with each other in real time. The animated repre-
sentatives of absent group members (Section 20.4) do not serve only as a means of
visualizing the implications of recommended solutions for the absent members. In ad-
dition, each member can grant her representative a certain amount of authority to accept
proposals during interactions with another group member. For example, in Figure 20.9,
Tina’s representative states that she cannot accept the current proposal. If instead the
representative had accepted it and the same were true of Ritchie’s representative and
the current user Claudia, the proposal would have been treated as finally accepted, even
though Tina and Ritchie had not really seen it.
20 Recommendation to Groups 623
20.5.2 Possible Extensions to Existing Systems
Since in most applied scenarios the overhead of animated agents would be too great,
we should consider how functions such as those of the TRAVEL DECISION FORUM can
be realized in a lighter-weight manner. For example, a system like POLYLENS might
allow each member to specify that they are willing to go to any movie whose predicted
rating for them is above some threshold (e.g., 4 stars out of 5). In that case, the system
could present not only recommendations but also a subset of the recommended movies
that can be decided on without further consultation; a designated group member could
then go on and buy tickets for any of these movies. If it is assumed that each group
member will view the recommendations before a decision is made, a procedure could be
introduced that allowed the group members to vote for movies among the recommended
ones, also indicating which particular ones they are willing to accept. In this case as
well, it could be agreed that a designated group member could make the final decision.
As the reader may have noticed, this type of voting mechanism can in itself be
viewed as a simple recommender system that makes use of explicit preference specifi-
cations and helps the group members to choose among the recommended options. And
in fact we could apply all of the concepts introduced so far to this “recommender”, con-
sidering, for example, whether the group members should be allowed to see each other’s
votes (cf. Section 20.2), how the votes should be counted and weighted (Section 20.3),
how the results of the voting should be presented (Section 20.4), and even how the re-
ally final decision ought to be made (the present section). Fortunately, we are not faced
with an infinite regress, since the recommendation problem that we are dealing with
now—choosing from among a small number of recommended items—is considerably
simpler than the original problem, and simple solutions may be quite adequate. For ex-
ample, suppose that for the original recommendations an aggregation function was used
which gave especially high weight to particular group members (e.g., the visitors from
out of town). It may not be necessary for the final voting mechanism to be biased in
their favor as well, since the set of recommendations itself will already contain mainly
films that the guests will like; and the hosts are likely in any case to vote in a way that
takes into account the greater importance of the guests.
More generally, decisions are often made in several stages, and in each stage a
different type of decision making procedure may be applied. Issues that require a great
deal of attention in one stage may be simpler to deal with in another stage.
20.6 General Conclusions
20.6.1 Conclusions Concerning Group Recommender systems
This chapter has shown that the differences between recommending for groups and
recommending for individuals are more numerous, important, and complex than one
would tend to think at first glance.
1. New methods of acquiring information about users’ preferences are available,
especially when group members specify their preferences explicitly; but the interpreta-
tion of explicitly specified preferences can be less straightforward than in the case of
individual users.
624 A. Jameson and B. Smyth
2. The process of selecting items to recommend for the group as a whole can involve
much more than the application of numerical formulas, and the appropriateness of the
potentially applicable methods depends on various aspects of the application setting.
3. An explanation of the considerations that underlie a recommendation can take
many different forms and convey many different types of information, which can be
processed further by the group members in their efforts to do justice to considerations
that could not be taken into account by the system.
4. The process of arriving at a final decision can require communication and nego-
tiation, which can be supported in various ways by the system, depending on the nature
of the setting.
Because of the many specific questions raised by these differences between rec-
ommendation for groups and for individuals, it is all too natural for designers and re-
searchers to focus on one or two of the differences, implicitly adopting with respect
to other issues a default solution that may be far from optimal for their particular set-
ting. We hope that, by accumulating and organizing ideas and experience concerning
the most important differences, this chapter will enable designers and researchers to do
justice to all of these differences in their own work, either by adopting ideas that have
already been developed or by working out new solutions of their own.
20.6.2 Implications for Other Types of Adaptive Web-Based System
Even more generally, just about any type of system that adapts to its users can be seen
in some sense as a recommender system, and it may be natural to extend it for adap-
tation to groups of users. For example, a system for personalized information access
(cf. the chapter by Gauch, Speretta, and Micarelli [13]) can be seen as “recommending”
particular documents to a user; and it is reasonable to adapt to groups of cooperating
information-seeking users. Either explicitly or implicitly, we will then have to deal with
issues such as the specification and aggregation of preferences of the various group
Similarly, a system that offers adaptive navigation support (cf. the chapter by
Brusilovsky [4]) can be seen as recommending moves within a hyperspace; and it is
natural to consider groups of navigating users (as is in fact done in LET’s BROWSE).
With systems that aim to encourage and support collaboration (cf. the chapter by
Soller [38]), one of the many functions is that of “recommending” to a set of potential
collaborators courses of action that are predicted to be beneficial to them. From a group
recommendation perspective, issues that arise include those of (a) how to select a form
of collaboration that best takes into account the possibly diverging goals and preferences
of the potential collaborators; and (b) how to convince the potential collaborators that
they would in fact benefit from the proposed collaboration—or how at least to enable
them to devise some form of collaboration that they consider more suitable.
Turning to the more domain-specific topic of systems for health care (cf. the chapter
by Cawsey et al. [6]), consider the example of a system designed to persuade members
of a family to adopt healthier eating habits: This type of intervention can be seen as in-
volving recommendations to a group of users who have different roles and preferences.
Most of the issues discussed in this chapter take a more complex form in this type of
setting than in the application settings discussed so far.
20 Recommendation to Groups 625
In the light of these and many other possible examples, it seems natural that group
recommender systems should attract increasing attention not only from designers and
researchers who are interested in this particular type of system but also from those who
work with other types of adaptive web-based systems.
Acknowledgments. For their support for the preparation of this chapter and of their
own relevant research, the authors are grateful to the following sources: for Anthony
Jameson, the German Ministry of Education and Research (BMBF) (projects MIAU
and SPECTER); for Barry Smyth, Science Foundation Ireland under grant 03/IN.3/ I361
and the Informatics Research Initiative of Enterprise Ireland. The anonymous reviewers
of this chapter provided valuable feedback that led to substantial improvements.
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... One of the most important elements of discussion when developing a GRS is the choice of the aggregation strategy. Different aggregation strategies and approaches to group recommendation are discussed in [18]. The evaluation of the group recommendation algorithm is essential to identify its usefulness. ...
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... Preference visibility Especially in the context of group decision making, the degree to which preferences of individual group members are made visible to other group members can have a significant impact on the construction of preferences . Group members want to see the preferences of other group members, for example, to have access to the opinions and knowledge of problem-specific experts in the group (effort-saving aspect (Jameson & Smyth, 2007)). Furthermore, knowing the preferences of other group members can also help to streamline the group decision process and avoid inconsistencies in individual preferences. ...
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User preferences are a crucial input needed by recommender systems to determine relevant items. In single-shot recommendation scenarios such as content-based filtering and collaborative filtering, user preferences are represented, for example, as keywords , categories , and item ratings . In conversational recommendation approaches such as constraint-based and critiquing-based recommendation, user preferences are often represented on the semantic level in terms of item attribute values and critiques . In this article, we provide an overview of preference representations used in different types of recommender systems. In this context, we take into account the fact that preferences aren’t stable but are rather constructed within the scope of a recommendation process. In which way preferences are determined and adapted is influenced by various factors such as personality traits , emotional states , and cognitive biases . We summarize preference construction related research and also discuss aspects of counteracting cognitive biases.
... Adaptive Radio (Chao et al., 2005) is a group recommender which selects songs to play in a shared environment. As suggested by (Jameson & Smyth, 2007), if group recommendations are generated paying attention not to suggest items which will be disliked by any group member (i.e., adopting the so called least misery strategy (Masthoff, 2004)), a very subtle differentiation among various degrees of liking may be unnecessary. In such cases, simply concentrating on negative preferences would be more effective 3 . ...
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... Such a case is group recommendation systems. Group recommendation systems recommend items to groups of users as opposed to a single user, for example a movie to a group of friends, an event to an online community, or a excursion to a group of tourists [4,46]. In this case, we have different types of consumer fairness, since now the consumer is not just a single user. ...
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We increasingly depend on a variety of data-driven algorithmic systems to assist us in many aspects of life. Search engines and recommender systems among others are used as sources of information and to help us in making all sort of decisions from selecting restaurants and books, to choosing friends and careers. This has given rise to important concerns regarding the fairness of such systems. In this work, we aim at presenting a toolkit of definitions, models and methods used for ensuring fairness in rankings and recommendations. Our objectives are threefold: (a) to provide a solid framework on a novel, quickly evolving and impactful domain, (b) to present related methods and put them into perspective and (c) to highlight open challenges and research paths for future work.
Recommender systems provide personalized content from various choices by mining users’ past preferences. Recommendation helps to overcome the information overload problem as the available alternatives consume a large amount of data. In a few of the applications, a group of people are involved in the process of generating a recommendation. This paper focuses on “automatically detected groups” formation for order and flexible preferences in group recommendation using locality-sensitive hashing. The MinHash technique is applied on a characteristic matrix to generate the signature matrix. The signature matrix is the reduced representation of the characteristic matrix and preserves the Jaccard similarity to a great extent. Locality-sensitive hashing is applied on the signature matrix to efficiently determine the users that are similar. Similar users will be the members of an automatically identified group. Therefore, the group members are maximally satisfied with recommended items. This work also studies the performance of benchmark clustering approaches in group formation. We experimented on real-world datasets and found that the proposed models to identify communities in group recommendation maximizes consensus among users in a group.
Recommendation systems (RSs) can establish a relationship between users and items and recommend the seemingly unrelated but actually interesting items to target users by utilizing their behavior. However, the recommendation performance of RSs is seriously affected by the issue of data sparsity because the information amount that each user involves is very limited with the continuous growth of both users and items. To solve the issue of data sparsity in RSs, a novel collaborative filtering prediction method is proposed, called multi-order nearest neighbor prediction (MNNP). The concept of multi-order nearest neighbors is an extension of friends of a friend. By using multi-order nearest neighbors, MNNP can not only expand the range of neighbors but also implement the neighbor propagation efficiently. For a target user, MNNP needs to search its multi-order nearest neighbors and successively uses them to iteratively update the rating matrix. To show the procedure of MNNP, we illustrate it by an example. Extensive experiments on real-world datasets show that MNNP has a good performance.
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This paper overviews methods and techniques useful for building group-adaptive systems and presents an experience of building a system to give news adapted to different group of users in a public space. Starting from the analysis of limits of group modelling strategies and of problems to pass from a user modelling to a group modelling approach in the adaptation of system interaction, we suggest an update of the probabilistic group model to improve the interaction of groups of users with devices devoted to show news. In particular, we analyze the way to build a group model useful to improve the adaptation of a system that provides news on a video wall in a public space which is attended from a group of users with common interests.
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When people engage with information, they are often in so-cial groups. This applies, for example, in the case of museum visits, where people typically attend the museum and view the exhibits in small groups. This paper describes a proposed system GM, Group Modeller, which will create group models from a set of individual user models. We discuss the approaches and challenges in terms of common sub-models, collective models, group interaction models, and knowledge-based rea-soning across models.