Notation lor sub ia denotes the log-odds ratio for State i and age group a and is defined as the difference of two quantities, quantity1 minus quantity2. Quantity1 is defined as the natural logarithm of the ratio of predicted probability of substance use based on the 2000 and 2001 data for State i, age group a (pi sub ia at time 00 and 01), and 1 minus the predicted probability of substance use based on the 2000 and 2001 data for State i, age group a (pi sub ia at time 00 and 01). Quantity2 is defined as the natural logarithm of the ratio of predicted probability of substance use based on the 1999 and 2000 data for State i, age group a (pi sub ia at time 99 and 00), and 1 minus the predicted probability of substance use based on the 1999 and 2000 data for State i, age group a (pi sub ia at time 99 and 00).