A clinical trial is being planned, in which the primary outcome is binary (success/failure). There are two treatment groups (treatment/control). The aim of the trial is to show that the treatment is better. The outcome is bad, meaning that in the treatment group, we wish to have a smaller proportion.
The events of interest are rare, and assumed to be around 7% in the control group and 5% in the treatment group.
I did a lot of reading, and some people say that with low rate of events it's better to use the risk ratio (relative risk) over the proportion difference (with normal approximation). It's not clear to me why.
I wanted to ask, under the conditions described above, what is best: Fisher's Exact Test, Odds Ratio (with a CI or hypothesis testing of OR=1), or Relative Risk (with CI or hypothesis testing of RR=1)? Will I get similar results in all? Should sample size be roughly the same?
Thank you !