“…Since drug use is a highly-stigmatized set of behaviours, it is impossible to create random samples of drug users. Nonetheless, considerable knowledge has been acquired through non-random community samples in studies of drug users and their neighbourhoods by us (Friedman et al, 1989(Friedman et al, , 2007a(Friedman et al, , b, 2008Neaigus, Miller, Friedman, & Des Jarlais, in press;Rossi et al, 2008) and others (Andia, Deren, Robles, Kang, & Colon, 2008;Latkin, Forman, Knowlton, & Sherman, 2003;Latkin & Knowlton, 2005).…”