"How do online and offline sampling compare in a multinational study of drug use and nightlife behaviour?".

Background: Online sampling is widely used to recruit hard to reach samples such as drug users at nightlife events. We conducted the first study comparing differences in demographics, drug use and nightlife behaviour between an online sample of young adults engaging with the European nightlife scene, and an offline sample recruited at nightclubs and festivals in Europe.

Methods: Online participants who attended at least six nightlife events in the past 12 months were recruited using social media advertising (May-November 2017). Offline participants were recruited at nightclubs and festivals using a random intercept method (May-November 2017). Samples were compared with respect to age, gender, past year use of alcohol, cannabis, cocaine, ecstasy/MDMA and amphetamines, and past year attendance at nightclubs, licensed festivals, illegal festivals, pubs and house-parties.

Results: 6153 online and 3529 offline participants were recruited. When adjusting for differences in age and gender, online participants were less likely to have used each drug and to have attended illegal festivals, pubs and house-parties in the past 12 months. The online sample also used each drug and attended each venue, with the exception of nightclubs, less frequently on average than offline participants. Adjusted odds ratios (range 0.37 to 1.39) and regression coefficients (range -0.84 to 0.07) indicate that the majority of observed differences between the samples were of a small effect size.

Conclusions: Estimates of drug use and nightlife engagement are more conservative when using online sampling compared to venue based sampling. Observed differences were generally small in effect, indicating good overall representativeness when using online sampling in the European nightlife scene.