Sampling hidden populations. ‘A Cornell University sociologist has transformed the small world concept
of “six degrees of separation” into a scientific sampling method for
finding and studying “hidden populations,” from drug users to jazz
musicians.

‘There are no lists available or associations of runaway youths, for
example. But this sampling method takes advantage of the fact that
individuals in a group know each other. As we gather information during
the sampling process of referrals, we look at the degree to which people
tend to recruit those similar to them. Then, we can mathematically
correct for the non-randomness and project what the sample would have
been had there been no biases,” says Douglas Heckathorn, professor of
sociology at Cornell.’