The combination of survreg + gamma frailty = invalid model, i.e., the example
that you quote.
I did not realize that this had been added to the survreg help file until
very
recently. I will try to fix the oversight. Other, more detailed documentation
states that Gaussian frailty + AIC is the only valid random effects choice for
survreg.
Details: frailty(x) with no optional arguments defaults to "mle" as the
method
of choosing the variance of the random effect, via a calculation that is
specific to the Cox model.
Terry Therneau
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