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about the potential for bias during randomization of partici-
pants in clinical trials.1–5I designed the Chocolate Happiness
Undergoing More Pleasantness (CHUMP) study to compare the
effects of dark chocolate, milk chocolate and “normal” choco-
late consumption on happiness. The CHUMP study was a
double-blinded clinical trial, and it demonstrated the difficul-
ties associated with performing a truly blinded clinical trial.
he randomized controlled trial is the “gold standard” for
testing the beneficial and harmful effects of interven-
tions. There has been a growing concern in the literature
The CHUMP study was conducted over a 1-month period at a
tertiary pediatric health care centre. Between Oct. 15, 2006,
and Nov. 15, 2006, 144 adults were asked to participate in the
CHUMP study. Owing to the study’s popularity and the in-
sistence of the research ethics board, an additional 36 adults
were added to the study, for a total of 180 participants (108
women, 72 men).The research ethics board of the Chocolate
Consumption Society approved the CHUMP study.
The participants were randomized into 3 study groups.
Group 1 received 1 dark chocolate bar (50 g, > 90% cocoa)
each day, and group 2 received 1 milk chocolate bar (50 g,
20% cocoa) each day. Members of group 3 did not receive any
additional chocolate; however, they continued with their nor-
mal chocolate-eating habits.
Each participant rated their happiness before and after the
study using a visual analog scale (0 = unhappy, 10 = happy).
Participants were also asked to rate their health, global happi-
ness and personal work history (0 = worst, 10 = best). In or-
der to establish participants’ baseline chocolate-eating
habits, participants were asked the frequency of chocolate
consumption and the amount consumed on a daily basis to
give a total chocolate bar day-equivalent. The total chocolate
bar day-equivalent included both dark chocolate bar day-
equivalents and milk chocolate bar day-equivalents.
I anaylzed contingency tables with χ2tests, and I used analy-
sis of variance to test for differences in the mean chocolate con-
sumption among the 3 groups. No specific research funding
was obtained for the CHUMP study; however, Costco provided a
generous discount toward the purchase of the boxes of choco-
late consumed in the study.
The CHUMP study began with 60 participants in each group.
Despite my efforts to keep the study groups blinded, some of
the participants changed groups mid-study. Indeed, those in the
control group (who received no extra chocolate) decided to start
A clinical trial gone awry: the Chocolate Happiness
Undergoing More Pleasantness (CHUMP) study
Research of a holiday kind
The randomized controlled trial is the “gold standard” for
evaluating the benefits and harms of interventions. The
Chocolate Happiness Undergoing More Pleasantness
(CHUMP) study was designed to compare the effects of dark
chocolate, milk chocolate and normal chocolate consumption
on happiness. Although the intention-to-treat analysis showed
that participants who received either dark or milk chocolate
were happier than those who received no additional choco-
late, the actual-consumption analysis showed that there were
no differences between any of the groups. The reason for this
result is that many participants switched groups mid-study be-
cause of their personal chocolate preferences. Although the
CHUMP study was pleasurable, it demonstrated the diffi-
culties associated with performing a truly blinded clinical trial.
CMAJ • December 4, 2007 • 177(12)
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raiding the chocolate of those in the interventional arms (dark
chocolate or milk chocolate). Furthermore, participants in the
dark and milk chocolate groups traded chocolate based on their
individual preferences. The milk chocolate study group was the
most popular, as the number of participants in this group in-
creased from 60 at inception to 82 at completion.
Table 1 shows the characteristics of the study participants
at the beginning of the study. There was no significant differ-
ence in age or in the chocolate consumption habits of people
in the dark chocolate, milk chocolate or no chocolate groups.
Another problem that occurred during the study was the
holiday known as Halloween. Participants’ chocolate con-
sumption changed following Halloween, as some participants
increased their chocolate consumption by eating extra choco-
late that was intended to be distributed to children on Hal-
loween and by raiding their children’s loot bags for extra treats.
In the post-study survey, I attempted to identify any
changes in the participants’ eating preferences. The
intention-to-treat analysis (60 participants per group) and the
“actual-consumption” analysis are presented in Table 2. The
actual-consumption analysis was performed after adjustment
for those participants who changed study groups mid-study.
As shown in Table 2, the intention-to-treat analysis predicted
that participants in either “intervention” arm were more likely
to be happy than those in the control arm. However, after the
actual-consumption analysis was performed, there is no sig-
nificant difference in happiness between any of the groups.
The strong belief that chocolate consumption leads to more
happiness was not observed in the CHUMP study. A far more
important indicator of happiness appears to be “getting what
you want, when you want it.” As shown by the intention-to-
treat analysis in Table 2, participants in the dark chocolate or
milk chocolate intervention groups were far more likely to be
happy. However, once the participants who had changed
groups during the course of the study were accounted for,
there was no significant difference in happiness among the 3
groups. This suggests that people who were most unhappy
with the “no chocolate” intervention were more likely to
switch to the dark or milk chocolate groups.
There are several limitations of the CHUMP study. One
participant noted in the post-study interview: “My selection
into the ‘no chocolate’ group led to utter disappointment,
and I made an all-out attempt to get at least a few chocolate
bars for free.” Thus, participant randomization and subse-
quent recognition of being in the “no chocolate” group may
Table 1: Characteristics of participants in the CHUMP study
Study group; mean (SD)*
n = 60
n = 60
n = 60
Female, no. (%)
Dark chocolate bar-days
Milk chocolate bar-days
Total chocolate bar-days 0.41 (0.08) 0.41 (0.11) 0.48 (0.11) 0.54
*Unless otherwise specified.
Table 2: Intention-to-treat and actual-consumption analysis for CHUMP study participants’ self-rated happiness with various aspects
of their lives
Study group; change in happiness (post-study minus pre-study happiness) (SD)
Dark chocolate Milk chocolate No chocolate
Personal work history
n = 60 n = 60
n = 82
n = 60
n = 64
n = 34
Personal work history 0.03 (0.01) 0.05 (0.02) 0.02 (0.01) 0.96
CMAJ • December 4, 2007 • 177(12)
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have adversely affected the happiness measures.
Second, the fact that the study period overlapped with Hal-
loween may have adversely affected the overall consumption
of chocolate during the study, and this may have led to
Third, I made no attempt to control for other possible
sources of happiness, such as the other basic “Cs” of happi-
ness: candy, chips, cookies, caffeine and cola. Although con-
sumption of any of these Cs would have led to happier partici-
pants, it may have had additive or multiplicitative effects with
chocolate consumption leading to biased results.
In conclusion, the CHUMP study shows the pitfalls of run-
ning a clinical trial in 1 locale. Crossover and contamination
of the study groups are easily achieved when taste buds are in-
volved. However, despite the problems of running a clinical
trial, there is one advantage: the author has a year’s supply of
chocolate left in his basement. Ah, happiness!
Kevin Chan MD MPH
Pediatric emergency physician
1.Van Der Windt DAWM, Koes BW, Aarst MV, et al. Practical aspects of conducting a
pragmatic randomized trial in primary care: patient recruitment and outcome as-
sessment. Br J Gen Pract2000;50:371-4.
2.Hill CL, LaValley MP, Felson DT. Discrepancy between published report and actual
conduct of randomized clinical trials. J Clin Epidemiol2002;55:783-6.
3.Boutron I, Estellat L, Guittet L, et al. Methods of blinding in reports of randomized
controlled trials assessing pharmacologic treatments: a systematic review. PLoS
4.Gluud LL. Bias in clinical intervention research. Am J Epidemiol 2006;163:493-
5.Hrobjartsson A, Forfang E, Haahr MT, et al. Blinded trials taken to the test: an
analysis of randomized clinical trials that reports tests for the success of blinding.
Int J Epidemiol2007;36:654-63.
Competing interests: There are no conflicts to report. However, if any choco-
late company feels induced to supply me with a lifetime supply of dark
chocolate, I would hardly refuse.
Acknowledgements: I greatly appreciate the low prices at Costco that al-
lowed for the cheap purchase of chocolate used in this study.
Disclaimer: For anyone who takes these data and analyses seriously, I also
have some Florida swampland for sale.