Comparing two different approaches to measuring drug use within the same survey

Respondents to the National Population Health Survey in Canada (1996 97) were asked two types of questions about drug use that allowed a comparison of the responses. The first question was about self-reported drug use categories: "In the past month, did you take [e.g., antidepressants]?" The second asked about specific drugs: "What specific medications did you take over the last two days?" Responses to the latter were coded according to the main chemical entity and then grouped in specific drug product categories similar to the first question's self- reported categories. The two sets of drug use categories were cross-tabulated for the 62,588 respondents who were 20 years of age and older. The proportion of persons who reported taking specific drugs who had not previously answered "yes" to the question related to the corresponding self-reported drug use category ranged from a low of 4.8% for insulin/oral hypoglycemics to a high of 43.7% for narcotic analgesics. Various reasons for these discrepancies are discussed. A series of logistic regression models relating the discrepancies to respondent characteristics shows that there is no clear pattern of variables associated with the discrepancies. These results show that surveys should be carefully planned to reflect the type of information needed.