A comparison of different survey periods in online surveys of persons with eating disorders and their relatives.

Evaluations of Internet-based interventions offered to patients with mental or psychosomatic illnesses are mostly carried out in the form of online surveys. This study tested whether modifying this time interval brings about changes in the respondents' response behavior and the make-up of the sample. Two survey strategies were compared: a postblock survey (with a variable time interval between the Internet-based intervention and the online survey) and a postspot survey (time interval standardized to 90 days). In the postspot survey, it was possible to prove a higher response rate (39.9%) than in the postblock survey (28.0%) as well as a higher number of questionnaires completed correctly and in full. No differences were identified in the basic characteristics of the random samples. It was possible to demonstrate that the quality of the online survey could be improved by standardizing the length of time. Standardization of the time difference implies a controllable degree of influence on the quality of the evaluations.