Purpose: While piloting for a study involving complicated trade off questions, we were confronted with the fact that many respondents were not able to understand the task they were asked to perform. Therefore we decided to adapt our recruitment procedure in order to try to select those respondents with adequate cognitive abilities for performing the trade off task.

Methods: In our initial design, consecutive patients from an outpatient clinic for heart failure were invited to participate after they had visited their physician, and a sample of elderly persons from the general population was to be addressed by means of a short invitational letter. In our adapted recruitment procedure, all respondents were invited to participate through a detailed information letter including an example of a trade off question. The trade off comprehended a choice between 2 patients, for whom only 1 treatment was available. If respondents did not want to make a choice, they could ‘draw lots'. All respondents were asked to return the answering form, and if they were not willing to participate, they were requested to explain why.

Results: While piloting, we found that 12 out of 15 interviews failed because the respondents were not able to (completely) grasp the trade off task. In total 337 people were invited according to the adapted recruitment procedure (preliminary response data). 186 returned the answering letter (55%), of whom 116 were willing to participate (62% of responders). 108 people were actually interviewed, of whom 102 (94%) successfully: 5 respondents did not understand the questions, and 1 refused to answer them. Of the 71 responding people who did not wish to participate, 24 (34%) stated that this was because they did not want to take a stand in the problem they were confronted with, while 3 (4%) declared that their inability to understand the question was the reason.

Conclusions: It is possible to efficiently select respondents who have adequate cognitive abilities for performing a complicated, abstract trade off task by means of using an invitation letter with an example of a trade off. However, as a consequence of this recruitment procedure, we found people, who do not want to make a trade off, to be underrepresented in our study population.