INTERNAL VALIDITY ANALYSIS - A METHOD FOR ADJUSTING CAPTURE-RECAPTUREESTIMATES OF PREVALENCE

The authors propose a method for adjusting results of log-linear multi -source capture-recapture estimates of total population. The method co mpares the totals in some subpopulations of known size with estimates derived from various capture-recapture approaches to these subpopulati ons. The authors term such an approach an ''internal validity analysis .'' Trends in the ratios of the estimates to the known true values of these subpopulations provide a plausible indicator of the bias of some types of estimates of the total population especially when underlying assumptions of the methods used have not been met in analysis of the total population. The authors apply this method to published data on a n open population of injection drug users that had been previously ana lyzed with a standard capture-recapture analysis as if it were a close d population. Internal validity analysis suggests that the size of thi s population is about 15% greater than that previously estimated.