When individuals are allowed to migrate in or out or there are deaths or births in the area, the population is termed as "open population". The primary assumption of capture-recapture is that the population under study is closed, that is there is no birth, death or migration. In another words, the very sample population is sampled by different sources. The closed population assumption will not be a problem in a study for investigating the incidence of a certain disease or injury, as in incidence study we consider the number of new events in a fixed time point. However, this will be a problem in a prevalence study as with serious conditions people tend to migrate to seek for cure. In addition, it is likely that the sampling or capturing procedure in a prevalence study takes over extended period of time and birth, death or migration may affect the composition of the population under study.