I'm doing a clinical trial and I'm comparing a certain number before treatment and after treatment. I recorded the numbers for each patient before treatment and after treatment and take the difference(after-before). But somebody commented that in the previous studies, it is possible that the study was conducted on extremely selected limited subjects, so there is no guarantee that the same parameter is observed as a result of the study. Therefore, we have to consider a more conservative way, like using ratio or percentage instead of the difference.
My question is, do I have to change the sample size here? I calculated the sample size using the sample size formula using type I error and type II error. I personally don't think the sample size needs to be changed, but I would love confirmation.