Incentives increased return rates but did not influence partial nonresponse or treatment outcome in a randomized trial

Objective: Our aim was to analyze monetary incentives and shortening the questionnaire as means of increasing response rates in a mailed follow-up survey 1 year after inpatient psychotherapeutic treatment. Additionally, effects on partial nonresponse and the assessment of treatment outcome were examined. Study Design and Setting: In a 2 x 2 factorial design, a sample of 3,825 patients was randomized to the two following interventions: (1) receiving a prepaid monetary incentive or none; and (2) getting a short or a long questionnaire. Treatment outcome was measured prospectively by a self-assessment instrument for psychopathology. Results: When using incentives, the response rate significantly increased by 7.3% (95% confidence interval [CI] 2.6-11.9%). Receiving a short questionnaire led to an augmentation of the response rate of 3.7% (95% CI 0.9-8.3%), which was not significant. The corresponding odds ratios were significantly increased for monetary incentives (1.36; 95% CI 1.30-1.88), and when abridging the questionnaire (1.15; 95% CI 1.01-1.31). However, partial nonresponse and treatment outcome were independent of the two factors. Conclusion: Incentives and a shorter questionnaire led to higher return rates but did not affect partial nonresponse and self-report of treatment outcome in a randomized postal survey. (C) 2007 Elsevier Inc. All rights reserved.