As a hobby application, I want to estimate a Kaplan-Meier curve for "survival" of football referees, i.e., for how long they stick to this activity. I have data for when they started and, if applicable, when they quit, or alternatively, that they are still active.
Now, the problem is that digital records only started about 15 years ago. So if a referee quit and/or has started during the last 15 years or has been active for more than 15 years, I know that. What I do not see is referees who started, say, in 1990 and quit in 1998. Given the problem of many referees quitting early, this seems to introduce a bias due to missing data.
I could use only data for referees who started refereeing during the last 15 years, which would however throw away much data on long-serving referees.
Is there any appropriate way to deal with this situation?