Internal consistency and measurement equivalence of the cannabis screening questions on the paper-and-pencil face-to-face ASSIST versus the online instrument

BACKGROUND: Validated Internet-based screening tools for cannabis use and abuse are needed. The present study aimed to establish equivalence between the previously validated Alcohol, Smoking and Substance Involvement Screening Test (ASSIST) as a paper-and-pencil (PaP)-administered questionnaire and its online use. METHODS: Two groups of cannabis users took part in this study and the results were analyzed using structural equation modeling. One group consisted of 150 participants and was assessed with the ASSIST PaP questionnaire in a face-to-face interview (the PaP group). They were recruited from three settings: a primary health care outpatient clinic, a general psychiatric facility, and an ambulatory specialized addiction treatment facility. The other group (the Web group) comprised 1382 persons who answered the online version of the same questionnaire. This sample was drawn from people who naturalistically visited a website dedicated to helping people with cannabis addiction. RESULTS: The internal consistency was good for the online questionnaire (0.74) and high for the already validated PaP questionnaire (0.91). The Web group, however, had higher scores on cannabis use than did the PaP group. The results show support for configural invariance, meaning that the one-factor structure was preserved across groups, although measurement equivalence between these two survey modes was not achieved. However, when the Web group was split into two random subsamples, measurement invariance was demonstrated between them by cross-validation. CONCLUSIONS: Measurement equivalence was not achieved between the two survey modes. Nonetheless, subanalyses of the Web group demonstrated that the cannabis screening questions of the ASSIST can be used for online screening. Differences in ASSIST scores between samples may be due to the sensitive nature of the information surveyed, with possible underreporting in face-to-face interviews, or to the different characteristics of the Web group because of the specialized nature of the website.