Two self-report surveys of delinquent behavior, the National Youth Survey and the Monitoring the Future study, indicate different rates of prevalence for illegal behavior. Trends in the two series differ also, and this has been taken as evidence for differential validity between the two studies. Comparison of the two data sets indicates that difference between them could be attributable primarily to differences in sampling design, the administration of the surveys, and the wording of specific questions. There appears to be little support for the assertion that one data set is more or less valid than the other for measuring rates or trends in crime and delinquency.