BACKGROUND: Various surveys now ask respondents to describe their most recent purchase of illicit drugs, as one mechanism through which market size can be estimated. This raises the question of whether issues surrounding the timing of survey administration might make a sample of most recent purchases differ from a random sample of all purchases. We investigate these issues through a series of questions which ask about the three most recent purchases, and about drug use. METHODS: Data were drawn from 688 respondents in the Melbourne Injecting Drug User Cohort Study across the period 2008-2013 and 2782 respondents to the Washington Cannabis Consumption Study in 2013. Responses to questions about the most recent purchases were compared to larger subsets of all recent purchases. RESULTS: For heroin, methamphetamine and cannabis no differences were found between the amount spent by participants on their most recent purchase and the average amount spent on three or more recent purchases. There were also no differences concerning the locations and types of deals, and the duration between consecutive cannabis purchases was the same for first and second most recent, and second and third most recent. CONCLUSIONS: Asking about the most recent purchase appears to be an economical way to learn about purchases more generally, with little evidence of substantial variation between the most recent purchase and other recent purchases reported by participants. In spite of consistent findings across our two surveys, further replication of the work reported in this paper involving other populations of users is warranted.
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