I am trying to analyze time-to-event data (time to completion of a task). Looking at the KM curves, there is a distinct behavioral change around 12 months. This makes sense, because at 12 months there is a policy requiring completion of the task to meet compliance.
I wanted to perform a cox regression, but this appears to violate the hazards assumption, and I was wondering the best way to handle this. I thought I might fit two models, one between 0-12 months and one 12+ months, but I don't know if this is the best way to handle this (and how one sets up data for this analysis in r). Maybe I would use time-variant covariates instead? I'm not sure how to set the data up for that either.