Does a Material Incentive Affect Response on a Psychotherapy Follow-up Questionnaire?

To investigate the effects of offering a small material incentive for participation in psychotherapy follow-up, 85 self-referred patients were randomly assigned to one of three experimental conditions: (a) promise of a lottery ticket conditional upon response to the follow-up questionnaire, (b) a lottery ticket unconditionally in advance of response, (c) no incentive. Patients in the unconditional reward condition took the longest time to respond and were the least likely to respond, especially those who were relatively more functionally impaired. Incentive conditions did not affect the quality of the data collected or the respondents' attitudes toward participation. However, there is some evidence that incentive conditions may jeopardize external and internal validity, by distorting the magnitude and/or direction of the effects of the independent variables under investigation.