If subjects smoked cannabis in a form other than a joint, e.g. pipes or bongs, they were asked to estimate the number of cannabis joints to which that would equate.
This conversion allowed cannabis use for all participants to be quantified in joint-yrs of use, with 1 joint-yr being equivalent to 1 joint per day for 1 yr. Subjects who had smoked ,20 cannabis joints in their lifetime were classified as nonsmokers and smokers of cannabis, respectively. A similar approach of determining lifetime use of cigarettes was employed to enable pack-yrs of cigarette smoking to be calculated, with 1 pack-yr being equivalent to 20 cigarettes per day for 1 yr. Subjects who reported smoking tobacco cigarettes occasionally but never daily, or daily at some stage in their life, were classified as nonsmokers and smokers of cigarettes, respectively. The questionnaire was piloted among ex-cannabis smokers