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How to ask about the use of new psychoactive substances to increase the validity of results in self-report prevalence surveys.

INTRODUCTION AND AIMS
New psychoactive substances (NPS) represent hundreds of novel compounds. However, the general public might not be familiar with the overarching term NPS. This can result in both under- and over-reporting of NPS use.


DESIGN AND METHODS
The study analysed the last-year prevalence of NPS use in an online survey conducted across I-TREND project countries (the Czech Republic, the Netherlands and Poland). Self-reported NPS use was assessed within two types of questions-a generic and a checklist question. We analysed prevalence for each question separately, incorporated the free-text probe 'other' that followed them, and combined the two questions into a conservative and an inclusive estimate.


RESULTS
Including free-text responses to the 'other' categories increased prevalence of NPS use (from 51% to 56% for the checklist question and 25% to 32% for the generic question). Taking an inclusive approach to estimating prevalence (i.e. indicating NPS use in either a generic list or from the checklist) yielded a higher prevalence estimate (60%, 95% confidence interval 58-62%), compared to a more conservative approach in which NPS use had to be affirmed by both questions (27%, 95% confidence interval 26-29%).


DISCUSSION AND CONCLUSIONS
Generic questions might lead to notably lower estimates of self-reported NPS use in comparison to checklists. However, creating relevant checklists is challenging and lengthy survey instruments have limitations. Further surveys might benefit from featuring a combination of the strategies used in this study-a single (generic) question involving a number of locally specific NPSs and a free-text 'other' probe.