The validity of self-reported drug use.

Response errors in self-reported drug questionnaires are evaluated, with particular reference to incidence and prevalence estimates. A simple model of response errors is defined, and some analytical results are obtained from this model. By assuming independence of response errors at different points in time, estimates for response errors are obtained from panel data. The results suggest that while the frequency of false negatives may be larger than the frequency of false positives, the latter error may nevertheless produce large errors in prevalence estimates, particularly if the prevalence rate is small.