Different perceptions of control: Applying an extended theory of planned behavior to legal and illegal drug use

Recent evidence has suggested that large numbers of university students both consume excessive quantities of alcohol and use cannabis. We designed this study to test the ability of an extended theory of planned behavior (TPB) to predict alcohol and cannabis use. The main focus of the study was the issue of perceived control, particularly the measurement of control beliefs, and the proposed distinction between self-efficacy and "perceived control over behavior." One hundred seventy-six students completed TPB questionnaires, 69% of whom were successfully contacted 1 week later to provide behavioral data. Findings provided support for the TPB as a predictor of alcohol-and cannabis-use intentions and behavior, although analysis of belief components showed that a multiplicative combination is not always optimal. Evidence from confirmatory factor analyses, underlying control beliefs, and differential prediction of the two behaviors provided support for the refined perceived control components. The study also provided beliefs that may provide targets for future intervention studies. Further research delineating the antecedents of TPB variables is required, as well as further investigation into the potential of the model as a framework for intervention.