The effect of using short versus detailed self-administered questionnaires on the estimate of illicit drug use among young adults.

We compared two study instruments in use in the Israel Defence Forces to ascertain the prevalence of illicit drug use: a short versus a detailed self-administered anonymous questionnaire. A relatively lower estimate of illicit drug use was found among subjects that answered the short questionnaire. The difference was noted for lifetime prevalence of illicit drug use for women but not for men (6.5% of women reported ever use of illicit drugs using the detailed questionnaire versus 2.1% of women using the short questionnaire, p = 0.002); for prevalence of infrequent illicit drug use (3.3% of men and 3.4% of women reported less than monthly use of illicit drugs using the detailed questionnaire versus 0.2% of men and none of the women with the short questionnaire; p = 0.004 and p < 0.001, respectively), but not for frequent use (i e monthly or more). There was no difference in the percent of subjects that admitted that they knew where to get drugs. We conclude that the short questionnaire in routine use for more than a decade is a useful instrument when compared with the detailed questionnaire, in that the yield of important information such as frequent illicit drug use is similar. This is achieved with the benefit of greater convenience and ease of application to large samples at a lower cost.