The external validity of results derived from ecstasy users recruited using purposive sampling strategies

This study sought to compare the patterns and correlates of 'recent' and 'regular' ecstasy use estimated on the basis of two datasets generated in 2001 in New South Wales, Australia, from a probability and a non-probability sample. The first was the National Drug Strategy Household Survey (NDSHS), a multistage probability sample of the general population; and the second was the Illicit Drug Reporting System (IDRS) Party Drugs Module, for which regular ecstasy users were recruited using purposive sampling strategies. NDSHS recent ecstasy users (any use in the preceding 12 months) were compared on a range of demographic and drug use variables to NDSHS regular ecstasy users (at least monthly use in the preceding 12 months) and purposively sampled regular ecstasy users (at least monthly use in the preceding 6 months). The demographic characteristics of the three samples were consistent. Among all three, the mean age was approximately 25 years, and a majority (60%) of subjects were male, relatively well-educated, and currently employed or studying. Patterns of ecstasy use were similar among the three samples, although compared to recent users, regular users were likely to report more frequent use of ecstasy. All samples were characterised by extensive polydrug use, although the two samples of regular ecstasy users reported higher rates of other illicit drug use than the sample of recent users. The similarities between the demographic and drug use characteristics of the samples are striking, and suggest that, at least in NSW, purposive sampling that seeks to draw from a wide cross-section of users and to sample a relatively large number of individuals, can give rise to samples of ecstasy users that may be considered sufficiently representative to reasonably warrant the drawing of inferences relating to the entire population. These findings may partially offset concerns that purposive samples of ecstasy users are likely to remain a primary source of ecstasy-related information.