Determination of the optimum period of interview for retrospective collection of data. An empirical study based on reported and documented outpatient contacts of depressive patients.

On the basis of reported and documented data on outpatient contacts of depressive patients, the reliability of data on the use of services, and the possibilities and sources of errors in the retrospective collection of data are presented and discussed. The results show an increase in errors the longer the period of inquiry is removed from the time of interview. The retrospective collection of data about ambulatory care more than 6 months ago has to be questioned from a reliability point of view. The correct dating of outpatient contacts succeeds substantially more reliably when questioned about periods of 3 months than about monthly periods, which leads to implications for the definition of patterns of care.