Background: Previous studies have shown that survey methodology can greatly influence prevalence estimates for alcohol and illicit drug use. The aim of this article is to assess the effect of data collection modes on alcohol misuse and drug use reports by comparing national estimates from computer-assisted telephone interviews (CATI) and audio-computer-assisted self interviews (A-CASI).

Design: Two national representative surveys conducted in 2005 in France by CATI (n = 24,674) and A-CASI (n = 8,111).

Participants: French-speaking individuals aged [18]-[64] years old.

Measurements: Alcohol misuse according to the CAGE test, cannabis use (lifetime, last year, 10+ in last month) and experimentation with cocaine, LSD, heroin, amphetamines, ecstasy, were measured with the same questions and wordings in the two surveys. Multivariate logistic regressions controlling for sociodemographic characteristics (age, educational level, marital status and professional status) were performed. Analyses were conducted on the whole sample and stratified by age (18-29 and 30-44 years old) and gender. 45-64 years old data were not analysed due to limited numbers.

Results: Overall national estimates were similar for 9 out of the 10 examined measures. However, after adjustment, A-CASI provided higher use for most types of illicit drugs among the youngest men (adjusted odds ratio, or OR, of 1.64 [1.08-2.49] for cocaine, 1.62 [1.10-2.38] for ecstasy, 1.99 [1.17-3.37] for LSD, 2.17 [1.07-4.43] for heroin, and 2.48 [1.41-4.35] for amphetamines), whereas use amongst women was similar in CATI and A-CASI, except for LSD in the 30-44 age group (OR = 3.60 [1.64-7.89]). Reported alcohol misuse was higher with A-CASI, for all ages and genders.

Conclusions: Although differences in the results over the whole population were relatively small between the surveys, the effect of data collection mode seemed to vary according to age and gender.