Establishment of a clear relationship is affected by the limited
number of studies providing relevant results, small numbers of
cases in reducers in some studies, limited dose–response data,
incomplete adjustment for baseline consumption, and the varying
definitions of reduction used. Interpretation is affected by the
likely inaccuracy of lifetime smoking history data collected in
case-control studies, and possibilities of bias in cohort studies if
reducers are more likely than continuers to quit during follow-up.