The model tested in the present study included measurement error as part of the beta coefficient estimate. This may have inflated the magnitude of the beta coefficient and thus contributed to the strong relationship between initiation and progression. This may explain partially why almost no unique environmental influence was estimated for quantity of cigarettes smoked and the estimate for unique environmental influence on quantity of marijuana use was non-significant, as the majority of unique environmental influences for progression, including measurement error, could be shared with the initiation of these substances use.
