I have two groups of subjects ('intervention' and 'control'), I am observing how many times people attend a hospital over a period of time and basically just want to know if the groups differ from each other.
What makes it complicated:
- The groups are of different size
- The subjects entered the study at different points in time so that the observation time is not the same for all (for some it's one year, for others just two months)
My question basically is what statistical test I have to use/how the data has to be transformed/preprocessed?
My first guess was to use a Poisson regression but I don't know how the dataset should be prepared for that. Do I need one row per subject/event/unit time?