I'm writing a proposal for a restrospective cohort study using registry data. Am I right to think of the registry data as the whole population?
If it is the population, then am I right in thinking there is no need to calculate required sample size (as is required for other cohort studies) or confidence intervals?
If it is a sample, what would be the study population, and how would I go about calculating sample size?
Additionally, with this in mind, how do I treat missing data? If there are 1000 heart transplant patients in a registry but only 500 of them have complete data (i.e. all the variables have values), do I treat these 500 patients as a sample? Furthermore, if it is a sample, it isn't random. Therefore is it worth calculating the confidence intervals from this actual sample size?