This depends on how good our methods are. If they can detect minor difference we need a small sample and short period of time. If the assessment method is less refined, we need a large sample size and randomly assign the patient to control and treatment groups. In spite of the random assignment persons may not be matched. For instance if the treatment arm has patients with larger GAA repeat numbers than those on the placebo, it may make a difference.
Age is also another factor.
This type of information will have to be adjusted and the statistics will have to be chosen, as you can see this type of study needs a lot of organization. When patients are assigned to control and treatment groups, especially in a retrospective study, we need a prospective placebo control study. You can end up having selection bias, and lose the comparability between the groups, groups not treated with the real drug and those that are. The persons that may take the drug in the retrospective study may be quite different from persons who did not. Certain groups of patients must selected out so that they are not compared to each other.