sample size: If I need to select three samples from a ....

If I need to select three samples from a very large population, how should I calculate sample size, if each sample has to represent one of the 3 different type of groups, each group having a 10% prevalance in the population?

Thank you!
msg

This is a common problem. It is easy enough when there are only two choices. What do you do with three or more? One way of thinking is to consider each group separately, just the one type verses all others. This reduces your problem to only two groups, the one you are interested in at the moment vs. everything else. Do this for each study group.

Note: Why do you have only 3 groups with 10% prevalance? Where is the other 70%?

sample size 3 groups

Thanks...i used n=pq/se*se formula ..so i get 90*10/3*3 so i get n=100 for each sample... how do i determine power and alpha
how do i determine sample size if i have to compare means of these groups.

the other 70% are groups with mixed characteristics eg; black, white, asian and all other of mixed races.