I had some questions about selection of patients and their time period for a retrospective survival analysis.
Let's say we want to perform survival analysis retrospectively on patients who had disease X during 2015 to 2019 and look at their survival at the end of 2020. Technically we could do survival with follow-up of up to 5 years because some patients will have follow-up of up to 5 years. But we believe that this would be incorrect because for the later patients, the follow-up is truncated (only 1 year of follow-up was allowed for patients in 2019) and censoring these patients at 2020 is not correct.
So if we include patients from 2015 to 2019, we can look at 1 year survival in 2020 because every patient has been allowed a theoretical follow-up of 1 year. If we want to check 2 year survival, we should only include patients from 2015 to 2018 (both censored and complete follow-up patients in this period). For 3 year survival, it should be period 2015 to 2017 and so on.
Then, we came across this opinion, that the above would be correct if we wanted to calculate the cumulative incidence/survival probability % for different groups in this cohort (so it would be accurate to get survival probability at 1 year for 2015-2019 patients, but 3 year estimates for 2015-2017 patients only). But if the aim is to look only at difference in survival (using log-rank test or HR in cox model) but not incidence/survival probability percentage, then all 2015-2019 patients could be included in survival analysis for all time periods.
What would be correct design- if all patients from 2015-2019 be included, could we do 1, 3, 5 year survival at 2020 or should we limit it to 1 year? Or the design changes according to the aim as mentioned above.