Let's summarize what we know so far. Jill is taking independent random samples from her target population of mice. Then she is measuring the tumor size once in each mouse for both a placebo group and a treatment group. Jill will then calculate the mean tumor size for each group. She is interested in testing the hypothesis that the difference between the means is zero versus the alternative that the difference is not zero. Now we have a clearer picture of how we should go about calculating sample size. -- ErinEsp - 07 Feb 2010