This is probably a basic question, but here goes: I have a data set with a group of at-risk patients (for illustrative purposes lets say for heart disease) who were enrolled in a databases, some were treated with a drug (lets say an anti-platelet drug) and they are followed to see if they die. I want to use Cox proportional hazards to model prognostic factors that affect survival time and see if drug exposure is one of them, how long do I follow them for before I censor?
A lot of the patients are enrolled at various times and have varying lengths of follow-up until either they are lost to follow-up or die (for example, some patients have 2 months of follow-up data because they were enrolled 2 months ago, others have 3 years of data because they have been followed that long but not died, some have died after 1 month of enrollment, and some are lost to follow-up after 1 month).
Do I just choose an arbitrary length of follow-up (such as 1 year since enrollment) before censoring anyone alive at that time point? Do I use the longest length of follow-up available and censor everyone before that time point who was lost to follow-up? Any advice would be really helpful, sorry if its a question that is answered elsewhere but I couldn't find the answer when I looked. Thanks!