I have a small cohort of people with a specific disease (n=300) and studying the incidence of a related disease (which is kind of rare and normally occur only as a subsequence of this main disease). I have longitudinal data for this cohort over 4 years and I am fine with the study design in terms of stats to use etc (knowing that there is a sample size restraint but I cannot do more with that, just addressing the limitation). My question is: knowing that this cohort is not exactly representative of the population (but probably the best I can get) how correct is to report incidence rate in my population of the 'related disease'? Is it a valid information or shall I just report frequency of new cases per year? And in case reporting the incidence rate is still a valid information how to approach to that? Standardizing? Calculating the person-time incidence rate? I guess it is a matter of sample size as well...
Many thanks in advance
Many thanks in advance