Drug abuse in Thailand has remained a serious health problem; its epidemic is still severe and widespread. Information on the number of illegal drug users is a benefit of the policy and the plan on narcotics control, to implement a reduction strategy, and to allocate resources to the health service. Nevertheless, it is difficult to measure the sizes of drug user populations directly because of many “hidden drug addicts”. Surveys, especially on the large
national scale, are unlikely to be the most efficient meth- ods due to a huge cost and manpower, the difficulty of receiving a true response, the problems of dealing with a hidden population and ethical issues.
Capture-recapture methods are a classical and useful tool to solve a hidden population problem and to estimate a total population size because it can estimate and adjust for the extent of incomplete ascertainment using infor- mation from overlapping lists of cases from two or more distinct sources [1]. Moreover, there are not only the conventional multiple sources methods but also the ap- proaches available based upon one source with repeated counts for each individual. In this study, a single source is considered from a surveillance system counting the number of times that a drug user went to a treatment in- stitution.
There were few studies in Thailand which used the capture-recapture method for estimating the number of drug users. Mastro et al. [2] estimated the number of HIV-infected injection drug users in Bangkok under two- sample sources of 18 methadone treatment centers and 72 urine testing police stations. Suppawattanabodee [3] used two sources of health treatment records and police arrestment records for estimating the number of drug users in Bangkok 2001. However, for one source with repeated count data, applications have been few relevant studies in Thailand. Böhning et al. [4] estimated the number of drug users in Bangkok 2001 by means of zero-truncated count mixture distributions. Viwatwong- kasem, Kuhnert, and Satitvipawee [5] projected the number of heroin users in Bangkok 2002 using the mix- ture of zero-truncated Poisson models. Note that the ze- ro-truncated Poisson mixture distributions are different from the mixture of zero-truncated Poisson models, at least the mixing distribution in both estimations.