Dynamics of participation in psychological studies. A meta-analysis.

Objectives: The main question of the meta-analysis is, whether the initial participation rate in psychological studies has decreased over time. Moreover, possible moderators of this time effect will be addressed: The design of an invitation letter, the topic, the data collection mode (online survey, telephone survey, face-to-face-interview, experiment), the burden of participating in the study (length, complexity) and the incentives given to participants. Eligibility criteria: Eligible studies for the meta-analysis have to report initial participation rates from empirical studies in psychology. Descriptives of the study design concerning the relevant moderators (invitation, data collection mode, burden, incentives) have to be given. Student samples will be excluded, because students are often obliged to participate for their studies and therefore, their motivation differs from other populations. Methods of synthesis: The outcome of interest will be the response rate in percent. As there may be several different treatments per study report, the data are hierarchical. Using the metafor package in R, two-level mixed effects models will be used. On the one hand, to meet the needs of the hierarchical data structure and the dependencies in the data. On the other hand, to enable testing moderator variables on the level of the report (e.g. publication year) and on the level of the intervention (e.g. data collection mode). To test the time effect, the influence of the year of data collection on the average response rate is tested. Afterwards, the characteristics of the survey design are tested as moderators for the time effect. other