  We present a new design and inference method for estimating population size
of a hidden population best reached through a link-tracing design. The strategy
involves the Rao-Blackwell Theorem applied to a sufficient statistic markedly
different from the usual one that arises in sampling from a finite population.
An empirical application is described. The result demonstrates that the
strategy can efficiently incorporate adaptively selected members of the sample
into the inference procedure.
