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Commitment Devices for Weight Loss: An Online Field Experiment
Last registered on January 05, 2017


Trial Information
General Information
Commitment Devices for Weight Loss: An Online Field Experiment
Initial registration date
November 11, 2015
Last updated
January 05, 2017 4:50 PM EST
Primary Investigator
University College London
Other Primary Investigator(s)
Additional Trial Information
Start date
End date
Secondary IDs
This paper examines how a commitment device – an arrangement to willingly bind future choices to help achieve a personal goal – might be applied to health behaviours for weight loss, building on Thaler and Shefrin’s (1981) planner-doer theory. It reports on a field experiment that tests the effects of a financial and reputational commitment device offered to clients of an online weight loss service. Findings suggest commitment devices may play a useful role as a public policy instrument, but design and targeting matter.
External Link(s)
Registration Citation
Savani, Manu. 2017. "Commitment Devices for Weight Loss: An Online Field Experiment." AEA RCT Registry. January 05. https://doi.org/10.1257/rct.942-2.0.
Former Citation
Savani, Manu. 2017. "Commitment Devices for Weight Loss: An Online Field Experiment." AEA RCT Registry. January 05. http://www.socialscienceregistry.org/trials/942/history/12910.
Sponsors & Partners

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Experimental Details
A field experiment was conducted over July 2013 to February 2014 in collaboration with an online weight loss company.

Following an online survey to collect baseline data, clients were randomly assigned to one of three experimental groups:
(i) Limited financial commitment (comparison group)
This group were offered a refund on the next 4 weeks’ subscription to Food Monitor. Not having made a payment, this group were hypothesised to have lower commitment to the weight loss goal. 98% complied with the refund offer, which amounted to £4 - £9.
(ii) Financial commitment (treatment 1)
This group were thanked for their time and asked to use the food diary as they normally would. As they continued to pay their subscription, this group continues to experience a financial commitment device.
(iii) Reputational plus financial commitment (treatment 2)
This group continued to pay their subscription and were asked to name a coach, someone encouraging and familiar with the individual’s weight loss goals, who might be contacted after 4 weeks by the researcher. This is hypothesised to add an additional repetitional commitment towards the weight loss goals. 41% of the 118 individuals offered this treatment complied by providing the name and contact details of their coach.
Intervention Start Date
Intervention End Date
Primary Outcomes
Primary Outcomes (end points)
The main outcome variables are self-reported weight loss at the end of 4 weeks and 12 weeks. The study also considers the role of commitment devices in improving usage of the online service.
Primary Outcomes (explanation)
Usage of the online service is constructed from number of times the user (i) made entries to the food journal, (ii) logged a weigh-in, and (iii) used the calorie counter tool to self-monitor calorie intake.
Secondary Outcomes
Secondary Outcomes (end points)
Secondary Outcomes (explanation)
Experimental Design
Experimental Design
The field experiment tests the effect of commitment on weight loss success, ranging from limited commitment (member of an online service but no monthly subscription), financial commitment (fee-paying member, business as usual), and reputational + financial commitment (fee-paying member who is also asked to name a coach to verify their success after 4 weeks).

The treatments are delivered through an online survey.
Experimental Design Details
Randomization Method
Via online survey tool which randomly displayed one of three possible messages to participants, determining which treatment they received.
Randomization Unit
Was the treatment clustered?
Experiment Characteristics
Sample size: planned number of clusters
Sample size: planned number of observations
364 individuals
Sample size (or number of clusters) by treatment arms
132 per treatment group, but the refund group was capped at 100 by the partner firm who subsidised the refunds.
Minimum detectable effect size for main outcomes (accounting for sample design and clustering)
Ex ante sample size calculations recommended 132 observations per treatment.
IRB Name
UCL Ethics Committee
IRB Approval Date
IRB Approval Number
Project 4518/002
Analysis Plan

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Post Trial Information
Study Withdrawal
Is the intervention completed?
Intervention Completion Date
February 28, 2014, 12:00 AM +00:00
Is data collection complete?
Data Collection Completion Date
February 28, 2014, 12:00 AM +00:00
Final Sample Size: Number of Clusters (Unit of Randomization)
364 individual
Was attrition correlated with treatment status?
Final Sample Size: Total Number of Observations
For weight loss outcomes: 187 eligible observations at 4 weeks, 162 at 12 weeks.
For self-monitoring outcomes: no attrition, all units had coded observations (no usage = 0)
Final Sample Size (or Number of Clusters) by Treatment Arms
Weight loss at 4 weeks: comparison group 38% attrition, refund group 31%, coach group 43% Weight loss at 12 weeks: comparison group 55% attrition, refund group 54%, coach group 59%.
Data Publication
Data Publication
Is public data available?
Program Files
Program Files
Reports, Papers & Other Materials
Relevant Paper(s)