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Rare Populations, Hidden Populations, and Link-Tracing Designs: What and Why?

Literature on snowball sampling and other ascending methodologies (intensive data collection methodologies) is widely scattered. In order to indicate a structure in the diversity of articles, snowball sampling and other ascending methodologies are embedded in the general concept of link-tracing methodologies. From this point of view link-tracing methodologies are considered as tools for analysing social structure and for locating members of special populations. By introducing a new theoretical concept for defining special populations, a subdivision is made in link-tracing methodologies as a tool to locate rare populations and as a tool to locate hidden populations.