I need help with the power calculations to determine the sample size of a randomized clinical trial. This is a relatively simple trial with two arms: an intervention arm and a control arm. Patients in the intervention arm will receive a dietary supplement while patients in the control arm will receive a placebo. This is a longitudinal trial starting at time zero, with patient visits at 6 and 12 months. It is my hope that the supplement will reduce the patient's risk of developing obesity.
I know from previous reports that at the 6 month visit, 25% of the controls should have developed obesity. Also from previous reports, at the 12 month visit, cumulatively, 45% of the controls should have developed obesity! I think that in the best case scenario, the supplement will reduce the risk of getting obesity to only 2.5% among patients in the control arm.
My question: How many patients do I need in the intervention arm and how many do I need in the control arm to detect this difference? If I wanted to detect a smaller difference (perhaps a difference of at least 5% between the two groups), how would that change my calculation?
I've found some websites with calculators: http://www.epibiostat.ucsf.edu/biostat/sampsize.html#proportions http://www.stat.ubc.ca/~rollin/stats/ssize/b2.html
But I'm not sure how to input my data or which calculator to use. Any help would be greatly appreciated!