How can one predict the sample size of subjects needed to pass the current FDA’s requirements for the preoperative skin preparation with the two recent additions (95% confidence intervals and 70% responder rates)? The purpose of the sample size formula presented by the Food and Drug Administration (FDA) is to detect if the test and positive control products are the same or different. It can also detect if the test product achieves greater reductions than the vehicle or the negative control. To check out what the sample size formula is CLICK HERE