A data frame of the same form as the input data frame, but 'cut' as
desired. Intervals exceeding window[2] will be given event = 0.
If the selection gives an empty result, NULL is returned, with no warning.
Arguments
dat
Input data frame. Must contain survival data.
window
Vector of length two; the age interval.
surv
Vector of length three giving the names of the central variables
in 'dat'.