Source Dependence

One of the critical assumptions of 2-sample capture-recapture method is the independence of the 2 sources. Positively dependent sources (e.g. hospital source and physician source, referral) and negatively dependent sources (e.g. mutually exclusive data sources based on geographic criteria) may cause either underestimation or overestimation (ref Brenner). Because of lack of information in the 2-sample method, it is impossible to check this assumption mathematically. Therefore, it relies on the users to make the judgement, intuitively, to see the sources meet the independence requirement. In the wildlife biology, 2-sample approach is the primary method and it has been proven to be useful. If only positively dependent sources could be found, the 2-sample estimate should serve as the lower bound of the true number of cases. However, if only 2 negatively dependent sources are available, it is not recommended to employ capture-recapture method as the over estimation can be extreme under some condition and the results will not mean anything.