I am a Stata novice (Stata -V14).

I have a data set which consists of 404 doctors each independently evaluating 60 patients and deciding if they have malignant disease or benign disease. I am then performing Cox regression on the diagnosis (i.e. evaluating to see if the diagnosis is prognostically significant - in my speciality, this is considered a surrogate of diagnostic accuracy). I want to account for the fact that although there are 24240 doctor-patient interactions (404 doctors * 60 patients), there are only 60 patients.

My first approach was to use the "cluster" option but a reviewer has suggested I should try to fit a "frailty model" along the lines of : stcox diagnosis, shared(patient_id)

I have tried this and get the following error:

"flat region resulting in a missing likelihood"

Having looked this error up, I cannot work out what is wrong - is it a problem with my data (i.e. there is an error in the data) or is it telling me that this operation cannot be performed (i.e. the data is fine but the analysis cannot be done). Without this information, I cannot start to rectify the problem. I would appreciate any help that can be given.