David Kanouse, PhD, Senior Behavioral Scientists at the RAND Corporation, et al., in the Feb. 1999 Journal of Sex Research article "Drawing a Probability Sample of Female Street Prostitutes in Los Angeles County," wrote:

“[L]ittle of what is known about the size of this population… has been derived from careful scientific study. Most studies of prostitutes rely on samples of convenience, typically recruiting in jails, STD clinics, and methadone maintenance programs. A few studies also include outreach recruitment of respondents in areas known for street prostitution…

The usual way to minimize sampling bias is through the use of probability sampling techniques. However, the nature of commercial sex work makes that approach especially difficult. Because prostitution is an illicit activity, registries or rosters of prostitutes are not available… persons in the general population who are willing to admit to such activity is inefficient and unlikely to yield satisfactory coverage of the target population.”

Feb. 1999