Notation pi sub 00 and 01 denotes the predicted probability of substance use in 2000 and 2001 and was computed as a ratio of two quantities. The quantity in the numerator is a sum of two terms. The first term is calculated as the product of the population projections in 2000 (N at time 00) and the predicted probability of substance use in 2000 (pi at time 00). The second term is calculated as the product of the population projections in 2001 (N at time 01) and the predicted probability pie of substance use 2001 (pi at time 00). The denominator is the sum of the population projections in 2000 and 2001, i.e., (N at time 00) and (N at time 01).