patients, such as the body‐mass index (BMI), blood glucose, and cholesterol level. Participants further answered a detailed questionnaire related to their socioeconomic background, additional health outcomes and preventive behavior. Moreover, the physician in charge assigned an individual weight‐loss target the participants were supposed to realize within four months after leaving the clinic. The target was chosen to lie between six and eight percent of the current body weight, which is about the critical threshold associated with beneficial health effects (Vidal, 2002).

The experiment consisted of four phases (see Figure 1).<sup>5</sup> After the discharge from the clinic (Rehabilitation Stay), participants entered the weight‐loss phase of four months, which was followed by a weight‐maintenance phase of 6 months and a 12‐month follow‐up phase. Two randomizations took place: one at the start of the weight‐loss phase and a second one at the start of the weight‐maintenance phase. The latter was not announced at the beginning of the experiment. Random assignment to the treatment and control groups in the weight‐loss phase took place after the discharge from the clinic. Stratified randomization by the clinics was carried out without replacement within blocks of 51 participants. Based on this randomization procedure, the participants were assigned to one of three groups with equal probability: either the control group or one of the two treatment groups. While members of the control group were not promised to receive any reward for achieving their weight‐loss target, members of the treatment groups were promised to receive up to EUR 150 (henceforth called _Group 150_ ) or EUR 300 (henceforth called _Group 300_ ).<sup>6</sup> All successful participants (irrespective of group assignment in the weight‐loss phase) were randomized again at the start of the weight‐maintenance phase. Participants were considered as successful if their achieved weight loss exceeded 50 percent of the targeted weight loss. Randomization (without replacement and stratification by the clinics) was used to produce three weight‐maintenance experimental groups with equal shares of participants. In this phase of the experiment, two weight‐maintenance premium groups were promised to receive up to EUR 250 (henceforth called _Group 250_ ) or 500 (henceforth called _Group 500_ ). Participants assigned to the control group were not informed about weight‐maintenance randomization. All participants were told to assure that their weight does not exceed the individually assigned target weight during the weight‐maintenance and follow‐up phases.

Members of the premium groups were paid the full bonus if they had reached or even exceeded their weight‐loss target at the end of the respective phase. Once the achieved weight loss exceeded 50 percent, they were rewarded proportionally to the maximum reward. As an example, consider a participant with an initial body weight of 120 kg (264.5 lbs.) and a target weight loss

> 5 We present a flow chart in Figure A1 in the Appendix.

> 6 The premium levels and the length of the treatment period are in the range of previous studies. Jeffery (1983), for instance, used premiums of US$ 30, US$ 150, and US$ 300, which correspond in terms of PPP to EUR 54, EUR 272, and EUR 544 in prices of 2011 (converted into present values of EUR based on the US consumer price index and the PPP exchange rate of 2011 provided by OECD (2012)).

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