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st: Cubic splines in stcox

From   Marc Peters <>
Subject   st: Cubic splines in stcox
Date   Fri, 4 Jan 2013 10:42:09 -0600

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,

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