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Caveats: Informative Censoring
Altering the risk set can be problematic
•Recall
that in order for survival estimates to be consistent, the risk set
in the sample must look like a random sample from the
population
–You should not selectively remove or change balls that
were (for their risk factors) particularly more likely or less
likely to be chosen
•If
you notice that I search the urn from top to bottom,
»Don’t
just change the balls sitting at the top of the urn
»Make
sure you stir the urn after each change