Hello,

From a registry, I wish to know the number of subjects necessary for the study.

The goal is to compare overall mortality between two groups, so I will do a comparison of mortality rates between the two groups.

However, I wonder if this calculation is appropriate because, finally, it is not a randomized trial but a cohort-type observational study. We do a measurement at the inclusion and we classify the patients in 2 groups then we look if they die more or less in one of the groups. I am not familiar with these studies or if the statistical methods are the same as for randomized trials.

Thanks for helping me.