st: Cubic splines in stcox

[Marc Peters <marcpeters32c@gmail.com>]

Dear Statalist users,


I am new to event history models and am estimating a Cox proportional
hazard model, with civil war as the dependent variable. the dependent
variable is measured at discrete intervals (yearly).

My basic model is estimated using the following syntax:

stset year, failure(cwar)

stcox dem gdp, cluster (country)

This model runs well, but I encounter problems when including cubic
splines, using this syntax:

mkspline sp_year=year, cubic nknots(3)

stcox dem gdp sp_year*, cluster (country)


When running the command I get the following message "flat region
resulting in a missing likelihood"


I am sure this is a very trivial question, but I would greatly
appreciate if anyone could help me in explaining what the problem is
and how I might fix it.

Thank you,


Marc
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