In order to examine the genetic contribution to problems among cocaine users, only individuals who reported having tried cocaine were included in this study, because experimentation is a necessary precursor to problem use. That is, the inclusion of non-users would make results unclear in terms of whether they identify influences on propensity to use, or influences on problem use severity, or both (Buckland, 2008). One consideration in measuring cocaine dependence is phenotypic noise due to environmental constraints on access to cocaine. The current sample was drawn from project centers in densely populated U.S. cities (such as Saint Louis and Detroit) and was specifically designed to over-sample individuals with substance-related problems. This suggests that availability of cocaine is a reasonable assumption for individuals included in the current sample.
