Evaluations of Internet-based interventions offered to patients with mental or psychosomatic illnesses are mostly carried out in the form of online surveys. Up until now, the methods used in carrying out these evaluations have seldom been systematically investigated. 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 post-block survey (with a variable time interval between the Internet-based intervention and the online survey) and a post-spot survey (time interval standardised to 90 days). In the post-spot survey it was possible to prove a higher response rate (39.9%) than in the post-block 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 standardising the length of time. Standardisation of the time difference implies a controllable degree of influence on the quality of the evaluations.