Estimating the prevalence of male clients of prostitute women in Vancouver with a simple capture-recapture method

Capture-recapture techniques are widely used to estimate the size of difficult-to-count human populations. Applications often focus on the overlap between two or more samples, but another type of data that is encountered in human studies involves only the number of times that particular individuals were encountered in the study period. We present a method for estimating the population size in this situation. This method is simple and technically accessible and allows for entries and exits by individuals and for a difference between probabilities of initial and subsequent contacts. We apply the method to arrest data on male clients of prostitute women in Vancouver.