14, and 17 years old. Diversity of substances used provides an important index of substance use severity (18). It also serves as a measure of substance involvement applicable across adolescence – diversity of substances used has the same meaning at all three ages of assessment. This measure is applicable even in the youngest age group because easily accessible substances like tobacco are included. Similarly, because less socially acceptable substances (i.e. amphetamines) are also included, diversity of substances endorsed retains variance in the oldest age group. By utilizing a longitudinal design and a measure with equivalent interpretability throughout adolescence, any detected age effects can be interpreted as true developmental trends.
