The EpiLink record linkage software: presentation and results of linkage test on
cancer registry files
A. Tittarelli, G. Tagliabue, A. Maghini, S. Fabiano, P. Crosignani, and P. Contiero
Cancer Registry Division, Istituto Nazionale per lo Studio e la Cura dei Tumori, Via
Venezian 1, 20133 Milan
Author for correspondence :
Cancer Registry Division,
Istituto Nazionale per lo Studio e la Cura dei Tumori,
Via Venezian 1, 20133 Milan, Italy.
Tel: +39 02 23902501; email: email@example.com
Cancer registry, Record linkage, Computer program, Follow-up methods
Objectives: Record linkage, the process of bringing together separately compiled but
related records from different databases, is essential in many areas of bio-medical
research. We developed a record linkage program (EpiLink) which employs a simple
mathematical approach. We describe the program and present results obtained testing it in
a linkage task
Methods: EpiLink was designed to be flexible with user friendly settings to tailor linkage
and operating parameters to specific linkage tasks, and employ deterministic, probabilistic
or sequential deterministic-probabilistic linkage strategies as required. The user can also
standardize data format, examine linkage results and accept or discard them. We used
Epilink to link a subset of cases of the Lombardy Cancer Registry (20,724 records) with
the Social Security file of the population (1,021,846 records) covered by the registry. The
linkage strategy was deterministic followed by several probabilistic linkage steps.
Results: Manual inspection of the results showed that EpiLink achieved 98.8 %
specificity and 96.5 % sensitivity.
Conclusions: EpiLink is a practical and accurate means of linking records from different
databases that can be used by non statisticians and is efficient in terms of human and
Record linkage is the process of bringing together separately-compiled but related records
from different databases (1) and is essential in cohort follow-up, clinical trials,
epidemiological studies, health service research and health service management
(2,3,4,5,6,7,8,9). The increasing use of large electronic health databases has accentuated
the requirement for automated systems of record linkage, since it is impractical to carry
out record linkage by hand. In fact essential or desirable record linkage projects have not
been actuated because, in the absence of automated procedures, the human and financial
resources required to complete such projects are prohibitively large (10).
The difficulties of record linkage vary with the characteristics of the data files being
linked. When a unique identifier is present (e.g. social security number), the linkage is
straightforward and consists of matching the records in the two data files by means of the
common identifier (1). However, unique identifiers are not always present, and this is
typically the case when the data set is assembled retrospectively. Moreover the unique
identifier may be subject to error. In such cases it is necessary to use other identifiers such
as surname, name, date of birth, etc. However this often leads to other difficulties, the
main ones being that personal identifiers are not unique to an individual and even a
combination may not uniquely identify an individual; they are also subject to errors, can
be missing, or may change with time (e.g. address).
To surmount these problems two approaches are possible: deterministic linkage and
probabilistic linkage (10,11). The deterministic approach only links records that perfectly
match in terms of the linkage items chosen. This is a simple low-cost procedure, however
it does not match records with errors or repetitions of individual records. The probabilistic
approach links records according to probabilistic rules designed to overcome problems
due to errors, including multiple occurrences of supposedly unique identifiers. However,
the use of probabilistic methods introduces considerable complexity into the linkage
process and requires the involvement of personnel with competence in statistics.
The most practicable approach at present is to implement a tailored strategy for each
record linkage task, deciding whether it should be deterministic or probabilistic, what
fields should be involved in the linking process, the rank of their importance, and the
statistical model used to identify the links between records. The choice of strategy will
therefore depend on the structure and quality of the database, user competence, and the
aims of the linkage. If the quality of data is high or a certain percentage of errors is
acceptable, the best linkage approach may be deterministic; otherwise probabilistic record
linking is necessary. It should be evident therefore that a probabilistic record linkage
program should be highly flexible in that its settings should be modifiable to suit the
linkage project being undertaken.
We have developed relatively simple probabilistic data linkage program, called EpiLink,
using a straightforward mathematical approach that can be understood by non-expert
users. Our aim was to produce a program that is cost-effective in terms of computer and
human resources, and that can be used as an alternative to more complex probabilistic
record linkage programs (12,13,14,15). The program was developed using database files
of the Lombardy Cancer Registry (LCR) and was intended to be:
− Flexible, permitting users to impose linkage settings according to the characteristics of
the records being linked.
− User-friendly, allowing operators unfamiliar with computer languages or statistical
methods to use the program efficiently via a familiar graphic user interface (GUI).
− Portable: implementable on various computer configurations and operating systems
with easy installation and maintenance procedures.
Record linkage is a central concern of cancer registries, whose principal mission is to
record all cancer cases in a defined population and collect and integrate information on
those cases from various sources including hospital records, mortality files and pathology
reports (16). The specific aim of the work reported here was to test the program in linking
a subset of the cases in the LCR (16,17) to the Lombardy region’s social security file in
order to check the life status of all cases not known to be dead, and to assess cancer site-
MATERIALS AND METHODS
To enhance the portability of the EpiLink record linkage computer program it was
implemented on client server architecture, and is capable of running on a stand-alone PC
or on more complex configurations, on top of Unix or various Windows operating
systems. The program has a simple installation procedure. When installed on a server it
can be used by more than one client simultaneously. It can perform multiple linkage
projects simultaneously following definition of multiple profiles. It works on existing
databases, and is also capable of accessing external tables and inserting them to an
existing database. The program can accept several types of data structure and working
environments so the data do not require modification before the program can use them. To
enhance ease of use the program was designed so that all options and operating parameters
can be set without changing the code.
The record linkage process takes place in a series of steps, each characterized by specific
user-defined options and parameters.
The choice of information items and statistical parameters to use is critical to the success
of probabilistic linkage. Running the linkage without a preliminary analysis of the
information in the databases is likely to result in a large number of erroneous linkages. We
resolved this problem by incorporating a procedure that allows trial linking on small
subsets of data. This allows the testing of hypotheses regarding the linkage, the heuristic
discovery of improvements, and provides information to decide which items are best
involved in the linkage process.
Options incorporated into the user interface allow the user to:
− Load external tables into the working database.
− Standardize data, for example by specifying date format.
− Select the items to be used in the linkage, and choose the parameters for comparison
− Execute the linkage.
− Check all linked records by a customisable interface
− Examine record linkage statistics.
− Discard trial linkage results.
Statistical approach to linking
We used a simple mathematical approach based on a family of functions called similarity
indexes which are widely used in taxonomy, ecology, genetics and text analysis
Similarity indexes are intuitive and simple to handle from the mathematical point of view.
The linkage is performed between two tables, the target table and the source table. The
target table has records which are to be matched with records in the source table. The
target table can have record duplications, the source cannot. A field x in the source is
matched to a field t in the target by means of the following similarity function which
affords a similarity value s(x, t) that expresses the percentage correspondence between the
values of the source and target fields:
s(x, t) = 2 | x ^ t | / ( lx + lt ) (x 100)
where lx and lt are the lengths of the compared strings and | x ^ t | are the characters in
common, taken in sequence.
Numerous string manipulators more or less similar to the s function are described in
literature (22). Some of these are more efficient than the s function in that they are able to
recognise probable misspellings for example, but introduce a degree of complexity that is
contrary to the aim of the present study.
Before computing the s function, EpiLink cleans the strings being considered of special
characters such as accents, apostrophes, commas, full stops (periods), and multiple spaces
between words, etc.
EpiLink does not specifically incorporate any search code algorithms such as Soundex or
Davidson that use various criteria (for example words pronounced alike) to suggest
matches. However, such algorithms can be applied to databases before using EpiLink.
The process of matching a source record to a target record involves assigning a weighting
to each field considered, and summing them as shown in the general formula below.
The need for weightings derives from the fact that the fields will have different error rates
and discriminating powers. Thus, if the error rate is high a low weighting would be
assigned and vice versa. The discriminating power of a field is inversely proportional to
the average recurrence frequency of values in that field. Thus the less often values recur in
a field, the more useful the field is for identifying a record, consequently it is assigned a
higher weighting (8).
The general formula for comparing records is :
S(x, t) = ∑i wi s(x i, t i) / ∑i wi (x 100)
where wi is the weighting assigned to the ith field. For each target record, the S value is
calculated for each record of the source and takes values from 0 (no similarity) to 1 (all
field values equal between the two records compared).
EpiLink leaves the user free to choose the weightings for each field, but the program
suggests weightings using the following formula derived from information theory:
wi = log 2 (1-eI )/ f I
where fi is the average frequency of values in the field, which can be obtained by analysis
of the source file or can be input by the user; and eI, is the error rate for a field which this
can be set by the user after a trial linkage or based on previous knowledge of the data set.
Consider for example the variable sex: if fi is ½ and the error rate is 1/20, wi is
log2 [(1.0-0.05)/ 0.5] = 0.93
To clarify the use of the S function, consider Table 1 which shows a sample of records to
Comparing the target record with the first record in the source file, the s function gives the
following values for each field: Surname = 1, Name = 1, Date of birth = 7/8, Sex = 1.
If we assign the weightings 4, 1, 4, and 1, to the fields, respectively, the value of the S
function for the whole target record is 95%. Comparison of the target record with the other
three records of the source file gives values for the S function of 93%, 87%, and 67%.
This algorithm only identifies similarities between the two records under comparison; it
does not calculate the frequency of a given value in a field in the files to be linked. Only
the average frequency of the values in the fields used for the linkage is used, to set the
weighting before starting the linkage process. In the above example the average frequency
of surnames was used, not the frequency of the surname Rossi.
The user must impose a threshold (acceptability threshold) for the percentage
correspondence between the source and target records used in the linking. If the
acceptability threshold is set at 80%, then records of the target table that are 80% or more
similar to a given source record are linked. An acceptability threshold of 100% is
equivalent to fully deterministic linkage; lower thresholds are probabilistic linkages. The
linkage procedure selects the record with the highest similarity exceeding the defined
acceptability threshold; if none are found at or above the threshold, no linkage is made.
Using this approach, however, erroneous linkages can be made. For example if the
program finds two possible links that differ by only a few percentage points, automatically
linking the one with the highest percentage similarity carries a fairly high risk of error. To
overcome this problem the program presents not only the most similar record, but also
those that are slightly less similar, the percentage range (tolerance threshold) being user-
If more than one record has a similarity above the acceptability threshold but the
percentage differences fall within the tolerance threshold, these records are placed in a
temporary link list, so that the user can choose the correct one by visual inspection. If the
record with the highest similarity falls above the acceptability threshold and there are no
other records within the tolerance set by the tolerance threshold, the link is considered
In the above example, if the acceptability threshold is set at 80% then the program
potentially flags three records (those with S function values of 95%, 93% and 87%). If
also the tolerance threshold is set at 5 percentage points, the program chooses the records
with S function values of 95% and 93%, and places them in the temporary link list, as the
difference between them is only two percentage points.
As noted, the linkage process takes place in several sessions. The first session is typically
a deterministic matching (100% similarity). The matches obtained in this session are
flagged and not considered in future sessions. In subsequent sessions probable links are
identified, progressively lowering the acceptability thresholds for items, and possibly
adjusting the weightings for different linkage items. Each session results in flagged links.
By opportunely setting the weightings, a session can select putative links in which the
error is in one or more items only, and this facilitates manual checking of putative links.
The efficacy of linkage is measured by the number of false negative (records left unlinked)
and false positive (records linked erroneously) (1). In the EpiLink system the way to
reduce false negatives is to lower the acceptability threshold, although this must increase
the number of false positives. To decrease the false positives the user can increase the
tolerance threshold, but this increases the amount of manual checking required to validate
the putative links placed in a temporary list.
The basic way that EpiLink operates is to compare each record from the target table with
all the records of the source table, selecting the most similar. However this is a time-
consuming operation when the databases are large, and to increase speed it is possible for
the user to set a particular field as a deterministic one. This means that a given record in
the target table is only compared with those records in the source table for which the
deterministically flagged field matches exactly. This operation greatly decreases the
number of comparisons that have to be made and reduces the computer resources and time
required to complete the linkage process. When setting this flag the string cleaning
function noted above is not applied.
The Lombardy Cancer Registry
The LCR is a population-based cancer registry (16,17), established in 1976, that covers
the population of the Province of Varese, Region of Lombardy, northern Italy. The
population is about 800,000. The information items collected by the registry for each
patient (cancer case) are general demographic characteristics, cancer site, tumor histotype,
according to the Standard International Classification of Disease (WHO, 1990), and date
of first diagnosis. The total number of personal data records contained in LCR is 88,410,
covering cancer incidence from 1976 to 1996.
The LCR is now archived on the Oracle 8.0 database, running on an IBM risk 6000
machine with 128M RAM. The operating system is Unix, but clients run on Windows NT.
The completeness of the LCR personal data used for linkage in the present study is 100%
for name, surname, sex and date of birth, 62% for place of birth.
The Social Security List
All those entitled to receive public health services in Italy (practically the whole
population) are listed in Social Security files. The computerized Social Security List (SSL)
for Lombardy was set up in 1987. A total of 1,021,846. records are present in the
Lombardy SSL. We worked with the portion of the SSL file pertaining to all residents in
the province of Varese during the period 1/1/1987 to 1/1/1997. The completeness of the
SSL personal data fields used in linkage procedure is 100% for name, surname, sex and
date of birth, and 57% for place of birth.
All LCR records of cancer cases incident between 1/1/1988 and 31/12/1996, and
ascertained as alive on 1/7/1999 were linked with the SSL. These records (total 20,724)
formed the target table, whereas the SSL was the source table. Preliminary analysis of the
data permitted us to decide which fields to use, the error rate, the discriminating power of
each field and the linkage strategy. Table 2 shows the fields chosen for linkage (surname,
name, date of birth, place of birth and sex) with source table error rates and average
frequencies of specific values; we used these to estimate field weightings for use in the S
Test linkages performed during the preliminary analysis suggested an optimal linkage
strategy as follows:
a) Run a deterministic linkage session to identify most links.
b) Run four probabilistic sessions with (i) acceptability threshold first set at 95% and
progressively lowered to 90%; (ii) weightings progressively changed according to
the error rate and discriminating power of the records left unlinked; and (iii)
tolerance threshold always set at 5% to detect false positives.
c) Run additional sessions (in the present case six sessions) to assist manual linking
of the small percentage of records not linked by the previous sessions.
To test the procedure, links produced by the first two sessions, where the acceptability
threshold was ≥95%, were checked randomly; all links produced in subsequent sessions
were also checked manually. Records not linked at the end of these sessions were searched
The results are shown in Table 3, according to session, session 1 being deterministic and
sessions 2-5 probabilistic. A total of 19,446 (93.8%) of the 20724 records were linked
after the first five sessions, with 90 records placed in a temporary list; 53 of the latter were
unlinked and 37 confirmed following visual inspection. Six stable links were also
discarded after manual inspection of the results. Of the 1188 records left unlinked by the
linkage process, 688 were linked by manual inspection, and 500 were left unlinked.
Table 4 shows data pertaining to the first five sessions. T+ refers to true links: records
linked by the deterministic linkage and checked manually in a sample of random cases, or
records linked probabilistic sessions and checked manually in each case. T- are the
unlinked records. E+ refers to the records linked by EpiLink, E- refers the records not
linked by EpiLink. From these data the specificity of the process was 98.8% and the
The 1188 records left unlinked by the first five session were linked manually with the aid
of six additional linkage sessions. These results are shown in table 5. Because we worked
with a low acceptability threshold, some of the putative linkages made in these extra
sessions were discarded following manual checking. However the process assisted the
identification of true links manually. The unlinked records left over at the end of the
process were persons (cancer cases) not present in the SSL.
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Our aim was to develop a record linkage system that was easy to use, portable and
flexible. Ease of use was achieved by using a simple mathematical approach and
designing a user-friendly GUI that allowed the user complete freedom to impose the
parameters of linkage process required without the need to modify the source code or the
statistical formulae. Flexibility was achieved by making it possible to define a linkage
plan with a variable number of sessions, with the additional possibility of imposing
different weightings and thresholds in each session. Our experience is that about 3-4 days
are required train a novice operator to use EpiLink, although this does not include
deciding what linkage strategy to use. The program is portable, in that it can be installed
on Windows-based PCs and servers, and also on Unix servers.
In the present study we investigated whether the simple mathematical approach
implemented in EpiLink produced acceptable linkage in terms of sensitivity and
specificity. The study was divided into two parts. In the first part (first five EpiLink
sessions) the linkages were automatic, although links flagged as temporary by the program
were checked manually. We also manually checked the all links provided by the third to
fifth sessions, to verify the accuracy of the linking process. As a result of this check, six
false positives were found. This low figure suggests it is possible to link without manual
checking, while still maintaining high specificity. If we had stopped after the fifth session
we would have successfully linked 96.5% of the records automatically.
The remaining 3.5% of records were linked manually in the second part of the study,
although we still used EpiLink to suggest possible links, thereby making the manual work