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

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Abstract

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.

Introduction

Over the last 15 years, the use of ‘ecstasy’1 has been documented through the administration of surveys to samples recruited in many countries, including Australia (Solowij et al., 1992, Topp et al., 1999), the United Kingdom (Curran and Travill, 1997, Forsyth, 1996, Parrott et al., 2001, Winstock et al., 2001), Italy (Parrott et al., 2001, Schifano et al., 1998), the United States (Cohen, 1995, Cottler et al., 2001, Klitzman et al., 2002, Mansergh et al., 2001, Peroutka et al., 1988) and Canada (Gross et al., 2002). These studies have provided important information about the patterns and correlates of ecstasy use among samples accessed through a variety of means. Recruitment strategies have included attendance at locations in which ecstasy use appears likely to occur, such as ‘rave’ (Gross et al., 2002), dance (Forsyth, 1996) and club (Curran and Travill, 1997) parties; advertising in media designed for patrons of dance events (Topp et al., 1999, Winstock et al., 2001); and the targeting of specific populations such as men who have sex with men (Klitzman et al., 2002, Mansergh et al., 2001), students (Peroutka et al., 1988) and clients of drug treatment programs (Schifano et al., 1998). The extent to which such samples are representative of ecstasy users in general, however, is not clear.

Survey research, such as that cited above, seeks to examine populations by selecting and studying samples chosen from the population of interest to determine the relative incidence, distributions and interrelations of sociological and psychological variables (Kerlinger, 1986). In other words, inferences about the entire population are drawn from the results of studies of samples. The ecstasy users recruited in the studies cited above were generally obtained through purposive sampling (characterised by the use of judgement and a deliberate effort to include presumably typical groups in the sample; Kerlinger, 1986) rather than the more desirable probability sampling (in which each sampling unit has a known probability of being selected so that inferences about the population can be derived from the sample with a measurable degree of precision; Lilienfeld and Lilienfeld, 1980).

Although many studies endeavour to recruit as wide a cross-section of users as possible, it is difficult to ascertain to which population(s) of users the results of the studies cited above apply, because illicit drug use is a ‘hidden’ and often socially stigmatised activity (Griffiths et al., 1993). The term ‘hidden population’ has been used in a variety of contexts (e.g., Spreen, 1992, Sudman et al., 1988, Wiebel, 1990). Regardless of exactly which definition is preferred, it is clear that the illegal and stigmatised behaviours of illicit drug users endow them with ‘low social visibility’ (Watters and Biernacki, 1989). This fact renders it impractical to define the parameters of the population of ecstasy users, and therefore to obtain a random, or probability, sample of that population. As a result of this ‘hidden’ nature of illicit drug use, the recruitment of samples representative of the general population is required to obtain results that can be generalised to the entire population of users. This requirement, as in research with users of other illicit drugs, has infrequently been met in ecstasy-related research.

Nonetheless, some studies have examined correlates of ecstasy use among representative samples of the general population. In Germany, a prospective study examined ecstasy use among two birth cohorts of adolescents, and found that those who had used ecstasy, stimulants or hallucinogens were more likely to be male and to also have used other illicit drugs (von Sydow et al., 2002). In Spain, compulsory military service allows the recruitment of representative samples of males, and a study of patterns of illicit drug use demonstrated that Spanish ecstasy users were more likely than non-users to have an extensive drug use history (Bobes et al., 2002). A representative survey of American college students found that, compared to those who had not used ecstasy, those who had were more likely to use other drugs, to engage in more social activities, to spend less time studying, and to have more than one sexual partner. Ecstasy users, however, were no more likely to be ‘academic underachievers’ or to be dissatisfied with their college education (Strote et al., 2002).

Previous Australian studies have been among the most timely and detailed examinations of ecstasy use (Solowij et al., 1992, Topp et al., 1999), but assessments of the extent of their external validity is constrained due to their reliance on purposive sampling. More recently, patterns and correlates of ecstasy use among Australia’s general population have been documented (Degenhardt et al., in press). The aim of the present paper is to examine the extent of concordance between key demographic and drug use variables obtained from two studies conducted in 2001: (1) a survey of a representative sample of 26,744 Australian adults aged 14 years or older (AIHW, 2002) and (2) a survey of 163 ecstasy users recruited using purposive sampling (Topp et al., 2002).

Section snippets

National drug strategy household survey

The 2001 National Drug Strategy Household Survey (NDSHS) was managed by the Australian Institute of Health and Welfare (AIHW) on behalf of the Commonwealth Department of Health and Ageing, conducted by Roy Morgan Research, and the data made available through the Social Sciences Data Archive at the Australian National University. The 2001 NDSHS (AIHW, 2002) collected data from 26,744 Australians on their awareness of, and attitudes and behaviour relating to, licit and illicit drug use. Along

Demographic characteristics

The similarities in demographic characteristics of ecstasy users from the two NDSHS samples and the purposive sample were striking (Table 1). All three samples reported a mean age in their mid-20s, there was little difference in the proportion of males, and the great majority of all groups had been raised in English speaking homes. The majority of all groups had completed high school, similar proportions of the three had gone on to complete either university or technical qualifications

Discussion

Survey research seeks to infer the characteristics of a target population by selecting and studying samples recruited from the population (Kerlinger, 1986). The theory of statistical inference demands that individual members of a target population have a known probability of being selected so that inferences about the population can be derived from the sample with a measurable degree of precision (Lilienfeld and Lilienfeld, 1980). Due to the ‘hidden’ nature of illicit drug use (Griffiths et

Conclusion

Although any research methodology incorporating non-random sampling necessitates caution when assessing the external validity of the results, the data compared in the present paper suggest that, at least in NSW, Australia, 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

Acknowledgements

This research was funded by the National Drug and Alcohol Research Centre. The Australian Institute of Health and Welfare manages the National Drug Strategy Household Survey on behalf of the Commonwealth Department of Health and Ageing. The 2001 Survey was conducted by Roy Morgan Research Pty. Ltd., and the data made available through the Social Sciences Data Archives at the Australian National University.

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