In the initial approach, no mention was made of the primary
risk factor of interest to avoid recruitment bias with either the
cases or the controls. To minimise response bias, the
interviewer did not state the specific research hypothesis and
took a detailed history of all well-known risk factors. Cannabis
exposure was measured as joint-yrs of cannabis use, which
combines both the intensity (amount and frequency) and
duration of use. This approach follows the current convention
for quantifying life-long cannabis consumption and recognises
the evidence that the risk of lung cancer with cigarette smoking
is related to both intensity and duration of use [29]. Recall of
the amount of cannabis smoked over a long period of time may
have been difficult for some subjects; however, this is likely to
have been similar for both cases and controls as subjects were
not aware of the study hypothesis.