Often, the number of opiate users in a city is not known. This lack of knowledge
hampers adequate policy making. However, the unknown size of the opiate using
population can be estimated with help of statistics. We review seven statistical models
and apply them to a real data-set. All models are based on a single data-set. This dataset
is incomplete in the sense that only opiate users who apply for methadone
maintenance are registered while all others are not. The only information which is
available in this data-set is the number of visits of uniquely identifiable persons who
are on methadone maintenance, plus a set of covariates like age, sex and so on for each
of these persons. Still, from this single and incomplete data-set inferences about the
total population size can be made. The estimates are compared and the pro's and con's
of the estimators are evaluated.