The principal strengths of this study include its use of a nationally representative adolescent sample with direct diagnostic interviews and the assessment of multiple stages of alcohol and drug use. Study limitations include its cross-sectional design, retrospective reporting for age at onset information, and the lack of assessment of alcohol or drug dependence without a history of abuse. However, although we would expect that only a small proportion of youth would be excluded in this latter reason, the conclusions are limited to the categories examined and may not be generalizable outside of the US adolescent population. Caution is also warranted in drawing parallels to findings from retrospective estimates in adult samples because such interviews may underestimate rates due to forgetting or other memory biases and may increase biases in estimates of age of disorder onset due to the longer period of recall.36
