BACKGROUND AND AIMS: The prevalence of cannabis use based on self-reports is likely to be underestimated in population surveys, especially in contexts where its use is a criminal offence. Indirect survey methods ask sensitive questions ensuring that answers cannot be identified with an individual respondent, therefore potentially resulting in more reliable estimates. We aimed to measure whether the indirect survey method 'randomized response technique' (RRT) increased response rate and/or increased disclosure of cannabis use among young adults compared with a traditional survey.

DESIGN: We conducted two parallel nation-wide surveys during the spring and the summer of 2021. The first survey was a traditional questionnaire-based one (focusing on substance use and gambling). The second survey applied an indirect survey method known as 'the cross-wise model' to questions related to cannabis use. The two surveys employed identical procedures (e.g. invitations, reminders and wording of the questions) SETTING AND PARTICIPANTS: The participants were young adults (aged 18-29 years) living in Sweden. The traditional survey had 1200 respondents (56.9% women) and the indirect survey had 2951 respondents (53.6% women). In both surveys, cannabis use was assessed according to three time-frames: life-time use; use during the past year; and use during the past 30 days.

FINDINGS: The estimated prevalence of cannabis use was two- to threefold higher on all measures when estimated using the indirect survey method compared with the traditional survey: use during life-time (43.2 versus 27.3%); during the past year (19.2 versus 10.4%); and during the past 30 days (13.2 versus 3.7%). The discrepancy was larger among males and individuals with an education shorter than 10 years, who were unemployed, and who were born in non-European countries.

CONCLUSIONS: Indirect survey methods may provide more accurate estimates than traditional surveys on prevalence of self-reported cannabis use.