A population-based control group was used rather than a
hospital-based control group as the latter is susceptible to
significant bias due to the many medical conditions associated
with cannabis use [30, 31]. In recognition of the low incidence of
lung cancer in the age group studied, four controls were studied
for each case to increase the power of the study. Whilst eligible
Maori control subjects were less likely to be interviewed, when
confined to non-Maori subjects, the results were very similar to
when they were included, suggesting that any differential
control response by ethnicity was not a major confounder of the
increased RR observed for lung cancer from cannabis use.