I have two groups of patients, treated at two time periods. I'm comparing survival. In the later group I have 3 years follow up time. In the early group I have of course access to longer follow up time.
What is correct?
Should I stop follow up in the early group at 3 years (1095 days), i.e. for a patient in the early group that has more than 1095 days survival I set survival to 1095 and treat them as not failed? Or shall I use the longer follow-up time in the early group when doing Cox regression?