Sampling a hidden population: noninstitutionalized drug users.

As the spread of AIDS continues, there is an urgent need to understand patterns of drug use among drug users. While it is recognized that probability samples are scientifically preferable, there is a need to balance the realities of field research targeting an elusive population with the need for a sound sampling plan. A purposive or targeted sampling plan can increase the possibility of obtaining a representative sample of drug users. Targeted sampling involves using several sampling methods. First, neighborhoods with high concentrations of drug users are identified within the community in order to identify clusters of individuals in the target population. Once the geographic areas from which to sample have been identified, stratification can be used as an effective way to get a representative sample from neighborhoods with differing drug use patterns. The sampling plan can be further strengthened by the use of quota sampling to ensure that subgroups of drug users are adequately represented in the sample.