Can a commitment contract promote weight loss? A field experiment in London
Last registered on March 09, 2017

Pre-Trial

Trial Information
General Information
Title
Can a commitment contract promote weight loss? A field experiment in London
RCT ID
AEARCTR-0000954
Initial registration date
November 20, 2015
Last updated
March 09, 2017 10:49 AM EST
Location(s)
Primary Investigator
Affiliation
University College London
Other Primary Investigator(s)
Additional Trial Information
Status
Completed
Start date
2014-01-06
End date
2016-05-31
Secondary IDs
Abstract
Time inconsistency – when we fail to follow our own plans – can be explained through a dual-self model where a farsighted ‘planner’ is overruled by a myopic ‘doer’ (Thaler and Shefrin, 1981). This lens is applied to weight loss and obesity. One solution is to bind our future choices to ensure we stay on the right path, using some commitment device. This field experiment tests whether a simple commitment contract to oneself can work in the context of a public weight loss programme to help obese and overweight individuals meet their goals. It further examines heterogeneity pathways and whether the commitment contract works better for some individuals more than others, and if so what this might imply for targeting commitment devices in public health programmes.
External Link(s)
Registration Citation
Citation
Savani, Manu. 2017. "Can a commitment contract promote weight loss? A field experiment in London." AEA RCT Registry. March 09. https://doi.org/10.1257/rct.954-4.0.
Former Citation
Savani, Manu. 2017. "Can a commitment contract promote weight loss? A field experiment in London." AEA RCT Registry. March 09. http://www.socialscienceregistry.org/trials/954/history/14815.
Sponsors & Partners

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Experimental Details
Interventions
Intervention(s)

Participants were randomised into two groups: comparison and treatment.The treatment was the offer of a commitment contract that they were invited to personalise by writing in their names, signature, and date. The contract was designed to have a certain degree of visual gravitas and formality, on card of A5 size that could be carried around in a handbag or satchel, or stuck on a fridge or wall. They were advised to keep it somewhere they would see it on most days, and to discuss it with friends and family if they wished but not with the other group members. Comparison group members completed a baseline survey and returned to the weight loss group, and asked to continue with the programme as normal.
Intervention Start Date
2016-01-20
Intervention End Date
2016-01-31
Primary Outcomes
Primary Outcomes (end points)

The primary outcome variable is change in weight at the end of the Shape Up programme. Other outcome variables of interest include:
• Attendance and completion rates over the programme (%).
• Whether there was any change in food and exercise behaviours.
Primary Outcomes (explanation)
The primary outcome variable weight loss is calculated as (start weight) – (end weight), with a positive number indicating weight loss. The earliest possible weight reading is taken, which is sometimes the second or third group meeting if participants were late to register for the programme. The final weight reading is taken from week 7 onwards. Participants with missing outcome data after week 7 were contacted to gather follow-up data. Some were not reachable, and interim data suggests an attrition rate of 20%, which is not out of line with published weight loss studies.
Secondary Outcomes
Secondary Outcomes (end points)
Secondary Outcomes (explanation)
Experimental Design
Experimental Design
I designed an experiment to run in collaboration with the ‘Shape Up’ weight loss programme, run by Camden Council’s Active Health team across various sites in North London. Shape Up is a 12-week, group-based programme that sets a 5% weight loss target for all clients. The group meets every week to discuss a different aspect of weight loss and lifestyle. Class tutors measure and record weight, facilitate group discussion, and coach individuals on their food journal and self-reflection.

The target population is the Active Health team’s client base. These individuals have been screened for eligibility based on their current BMI and home postcode, and might have been put forward for the programme either through a GP or self-referral. These same eligibility criteria apply to participation in the field experiment, and did not change during the trial. Under a normal Shape Up programme, the population have made no financial commitment, but arguably may be conscious of a mild reputational commitment to the group tutor.

The registration process involved a baseline survey with questions on lifestyle, wellbeing and health motivation using the 19-item scale from the Healthy Foundations Segmentation tool - a social marketing model intended to identify which of 5 health motivational groups an individual belongs to.
Experimental Design Details
Randomization Method
Randomisation done in advance by a computer using Stata 12.0. In some circumstances client lists were not shared in advance, and in this case a simple rule was applied based on the sum of digits in their date of birth to assign them to either treatment or comparison group.
Randomization Unit
Individual
Was the treatment clustered?
No
Experiment Characteristics
Sample size: planned number of clusters
N/A
Sample size: planned number of observations
200
Sample size (or number of clusters) by treatment arms
Aiming for 190-200 participants based on ex ante sample size calculations.
Minimum detectable effect size for main outcomes (accounting for sample design and clustering)
Sample size calculations (for individual randomisation designs) are reported below in 3 scenarios using weight loss in lbs as a main outcome variable. The Baseline, Moderate and High weight loss scenarios relate to there being a 1lb, 1.5lb, and 2lb per week difference between control and treatment groups. Each specifies 0.9 power and a two-sided test. I also test the effects of low or high standard deviation assumptions, using 2lb and 3lb respectively. Baseline (1 lb weight loss difference) 44 (low SD 1 lb) 190 (med SD 2 lbs) 380 (high SD 3 lbs) Moderate (1.5 lbs weight loss difference) 20 76 190 High weight loss (2 lbs weight loss difference) 12 44 96 Calculations suggest a sample size of 190 is needed in an individual randomised design in the baseline weight loss scenario with moderate standard deviation across the participants. These sample size calculations carry a degree of uncertainty, as they require estimates of the mean and standard deviation of weight loss for which data is not readily available; and they assume a full sample and no problematic attrition.
Supporting Documents and Materials

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IRB
INSTITUTIONAL REVIEW BOARDS (IRBs)
IRB Name
University College London Ethics Committee
IRB Approval Date
2013-12-16
IRB Approval Number
Project ID 4518/003
Analysis Plan

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Post-Trial
Post Trial Information
Study Withdrawal
Intervention
Is the intervention completed?
Yes
Intervention Completion Date
February 28, 2016, 12:00 AM +00:00
Is data collection complete?
Yes
Data Collection Completion Date
April 30, 2016, 12:00 AM +00:00
Final Sample Size: Number of Clusters (Unit of Randomization)
197 individuals
Was attrition correlated with treatment status?
No
Final Sample Size: Total Number of Observations
197 individuals
Final Sample Size (or Number of Clusters) by Treatment Arms
100 individuals in control group, 97 individuals treated.
Data Publication
Data Publication
Is public data available?
No

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Program Files
Program Files
No
Reports and Papers
Preliminary Reports
Relevant Papers