A multi center trial of a behavioral intervention to prevent HIV infection is being planned. High risk men and women who are not infected with HIV will be randomized to a special individualized intervention program or to "usual care". Usual care consists of a single counseling session. The randomization will be stratified by gender, number of sexual partners in the past year (<3, 3-5, 6+) and age (<20, 20-29, 30+)

a.How many different strata are there?

b. If separate randomization schedules are to be prepared for each of the 20 participating centers, how many schedules will have to be prepared?