Let's summarize what we know so far. Jill is taking random samples from her target population of mice. Then she is measuring whether or not each mouse gets the disease. Based on this information she will be able to calculate the sample proportion of mice in each group. Jill is interested in testing the hypothesis that the difference between the proportions is zero against the alternative that the difference isn't zero. Now we have a clearer picture of how we should go about calculating sample size. -- ErinEsp - 06 Mar 2010