There are many factors that may impact on the placebo response, resulting in obtaining wrong information and therefore making incorrect conclusions.
This article from NetDoctor is very pertinent and explains how even paying study subjects may influence results.
Cash incentives for weight loss studied
Last updated 10-02-2011
Obese people are “more motivated to shed pounds if they're paid”, reported the Daily Mail. However, “the weight might creep back on once the monetary carrot is gone.”
The story comes from a trial that investigated whether giving people a financial incentive to lose weight had any effect on weight loss. Over eight months, obese people given the incentive while taking part in a weight-monitoring programme lost significantly more weight than those on a monitoring programme alone. The benefit did not last, however, and nine months after the trial finished, there was no significant difference in weight loss between the groups.
As in previous studies, this research found that while financial incentives work during the period they are in place, they appear to have no effect on weight loss in the long term. The study is a useful contribution to the growing interest in “behavioural economics” – giving people financial incentives to make lifestyle changes.
Where did the story come from?
The study was carried out by researchers from Carnegie Mellon University, Pittsburgh, the University of Pennsylvania and the Philadelphia Veterans Affairs Medical Center, USA. It was funded by the US Department of Agriculture and the Hewlett Foundation. The study was published in the peer-reviewed Journal of General Internal Medicine.
What kind of research was this?
This randomised controlled trial investigated whether giving people a financial incentive to lose weight had any effect on weight loss. This type of study, in which participants are randomly assigned to two or more groups, with at least one receiving the intervention being tested (in this case, a financial incentive) and another (the comparison or control group) receiving standard care without the intervention, is considered to be the best and most reliable way of studying the effects of interventions. It eliminates the possibility of bias and can also assess the relative effects of different interventions.
The authors point out that obesity is a growing problem and that current weight-loss interventions have only modest success in helping people lose weight and keep it off. “Behavioural economics”, they say, is emerging as a possible way of modifying self-destructive behaviours such as those leading to obesity.
They also point out that in a previous study, which looked at financial incentives for weight loss over 16 weeks, participants regained a substantial amount of the weight lost during the intervention. In this study, the financial incentive was increased to eight months, to find out if a longer-term intervention would be more effective for losing weight and keeping it off. The researchers said they used the financial incentive in which the participants’ own money is put at risk because “loss aversion” (the tendency for people to give more importance to losses than gains) would magnify the impact of the incentive.
The researchers also labelled the last eight weeks of the study as a “weight-loss maintenance” phase for some participants, to see if this made them “less vigilant” in controlling their weight than those who viewed the entire study as a weight-loss programme.
What did the research involve?
The trial lasted eight months and consisted of a 24-week weight-loss phase, during which all participants were given a weight-loss goal of one pound a week, followed by an eight-week maintenance phase.
The researchers recruited 66 US patients from a veterans’ medical centre who were all obese, with body mass indexes (BMIs) of 30-40. Participants had to meet various eligibility criteria, such as being aged between 30 and 70.
The 66 veterans were randomly assigned to one of three groups:
One group took part in a weight-monitoring programme, involving monthly weigh-ins and consultation with a dietitian in which weight-loss strategies and goals were discussed.
A second group (called DC1) took part in the same programme, but were also given a financial incentive plan, in which they put their own money at risk if they failed to lose weight. Under this plan, participants were asked to contribute up to $3 daily to a fund that the researchers matched dollar for dollar. They were asked to report their weight daily by text. If they met their target weight loss at the end of the month, they would earn back their deposit, plus the matching funds from the researchers. Those who had not met their target weight loss lost that month’s deposit.
A third group (called DC2) took part in the same weight-monitoring programme and financial incentive plan, but were told that the period after 24 weeks was for “weight loss maintenance” (no such distinction was made in the other two groups). The aim of this was to see if people made less effort to control their weight if they thought they had passed the weight loss stage of the programme and were now simply maintaining their current weight.
In the first phase of the trial, all three groups were given a target of losing 24 pounds in the first 24 weeks. In the second phase, those who had attained this goal could choose a goal of losing 0, 0.5 or 1 pound a week while others who had not met their target had their weight loss goals reset. At the end of each month, participants received $20 for returning to the clinic to be weighed.
Deposit money that was forfeited by those who had not met their weight-loss targets was pooled and divided equally among the DC participants who had lost 20 pounds or more by the end of 24 weeks.
Researchers measured weight loss at the end of the 32-week trial. Weight was also measured again 36 weeks after the end of the trial. The researchers used standard statistical methods to assess the effects of the interventions.
What were the basic results?
The researchers found that:
At 32 weeks, there was no difference in weight loss between the two groups who were given financial incentives to lose weight. The average weight loss was 9.65 pounds in DC1 and 7.75 pounds in DC2 (no significant difference between the two). The researchers, therefore, pooled the results of both DC groups together.
Participants on the financial incentive plan (pooled results of both DC groups) lost significantly more weight than control participants. The average weight loss for those in the DC groups was 8.7 pounds, compared to 1.17 pounds for the control group (95% confidence interval of the difference in means: 0.56 pounds to 14.50 pounds. This means we can be 95% sure that this difference lies somewhere between a 0.56 difference [in favour of DC] and a 14.5 pound weight loss [in favour of DC]).
36 weeks after the 32-week intervention, most participants had regained the weight they had lost and the difference in weight loss between groups was no longer significant. The groups receiving financial incentives had lost, on average, 1.2 pounds (95% CI 2.58 pound weight gain to 5.00 pound weight loss) compared to the control group without incentives, who had lost on average 0.27 pounds (95% CI 3.77 pound gain to 4.30 pound loss).
Researchers also found that at 24 weeks, only 10.6% of all participants had attained the goal of losing 24 pounds and this rate was similar between the groups (9.1% of the control group and 11.4% of the DC groups, no significant difference). Similarly, at 32 weeks only a small proportion of each group had maintained a 24-pound weight loss (9.1% of the control group and 13.6% of the DC groups, no significant difference).
The average net incentive earnings over this period were $88.
Only 65% of participants returned to the clinic for their follow-up weigh-in at 36 weeks after the end of the intervention period.
How did the researchers interpret the results?
The researchers say this is the first study to show that deposit contract incentives can help people successfully keep weight off for 32 weeks. However, “substantial” weight was regained once the incentives stopped. They say that techniques to promote weight-loss maintenance once financial incentives stop is an important area of research.
This well-conducted study shows that monetary incentives (in this case, partly fear of losing money) can be effective in promoting weight loss as part of a structured programme, but that maintaining weight loss once the incentive is gone is more difficult.
The study had some limitations. The participants were mostly men, so it is unclear if the results would be the same for women. While the trial was randomised, which should have balanced out any differences between the groups, the researchers found significant differences between them in some areas. For example:
The average income in the DC groups was lower than that of the control group.
The DC1 arm contained a significantly higher proportion of smokers.
Participants in the DC2 group rated the importance of weight control lower than the other two groups.
Those in the DC1 arm rated their own health better than the other two groups.
Ideally, it would be preferable for groups to be balanced for such characteristics, which could potentially affect the results.
The study could also not be blinded, and participants knew whether they were receiving a financial incentive or not.
Finally, only 65% of participants returned for the follow-up weigh-in 36 weeks after the end of the trial. The researchers tried to minimise the effect of such a high drop-out rate by adjusting for this in their analysis, assuming that those who did not return had reverted to their weight at the start of the study. By doing this, they would be more likely to underestimate rather than overestimate the effect in those who did not return. However, a follow-up rate of above 80% would have been preferable.
These findings will be of interest to policy makers. The issue of whether asking people to gamble on possible weight loss is perhaps an ethical question that needs further debate.