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 means for all of the treatment groups are equal versus the alternative that they are not all equal. Now we have a clearer picture of how we should go about calculating sample size.
The sample size formula we use is based on the analysis of variance approach. This formula is rather complicated and will not be provided here. See Applied Linear Statistical Models by Kutner, Nachtsheim, Neter, and Li for a reference.
-- ErinEsp - 28 Feb 2010