If you specify the COVSANDWICH|COVS option on the PROC PHREG statement
and you are also using a STRATA statement, then PROC PHREG will produce
an incorrect covariance matrix resulting in incorrect sandwich estimates
of the standard errors in the Parameter Estimates table if any of the
strata levels have zero events.

This problem occurs with any TIES= option specification (Breslow, Efron,
Discrete, Exact) and occurs whether or not the AGGREGATE= option and ID
statement are used in conjunction with COVSANDWICH|COVS.

To circumvent the problem, remove the observations corresponding to the
strata levels for which there are no events before submitting the data
to PROC PHREG.

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