> Is the general convention to assign a value of 'yes' to the time dependent
> covariate on day A or day A+1?
The general convention is on day A+1.
This avoids several problems that can occur with assignment on day A. An
example is increase in PSA (prostate specific antigen) and the declaration of
progression of prostate cancer. If the measurement and the declaration occur
on
the same visit, it may well be that the measurement does not really predict
cancer, it predicts that the physician will decide that there is progression.
(And in any case, is it really interesting to be able to predict a medical
outcome that will happen tomorrow?)
In our group, we often move things to day A+3, due to the fact that a
patient's visit may span multiple days.
A counter-example would be where the covariate is administration of a
treatment, and one patient died of an allergic reaction a few hours after
recieving the drug. One would certainly count this death on the treatment arm.
But instantaneous cause-effect outcomes like this are unusual.
Terry Therneau
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