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Surveys of hard to reach populations (rare, no known sampling frames) have been, for some years, the object of methodological reflection. Various methods aiming at the production of an ‘extrapolable’ sample of these populations have been proposed: time-space sampling (TSS) or time-location sampling (TLS), respondent driven sampling (RDS), or the ‘capture - recapture’ method. After defining what a hard-to reach-population is, this article provides an outline of these various approaches before going on to briefly consider the papers contained in this special issue.
Marpsat et al. (Sun,) studied this question.