Notice: On April 23, 2014, Statalist moved from an email list to a forum, based at statalist.org.
From | Steve Samuels <sjsamuels@gmail.com> |
To | statalist@hsphsun2.harvard.edu |
Subject | Re: st: Cubic splines in stcox |
Date | Fri, 4 Jan 2013 20:49:40 -0500 |
Marc Peters <marcpeters32c@gmail.com>: No version of spline is going to "fix" your problem. Your model is failing precisely because you are fitting spline function(s) in year, which is the time dimension of the -stcox- model-. -stcox- is already fitting a flexible model in time, so the spline function(s) and year are likely to be nearly collinear. When this happens, the partial likelihood will not converge. For a simple example with n =3, see Example 1 of: Maximum Likelihood Inference for the Cox Regression Model with Applications to Missing Covariates Ming-Hui Chen, Joseph G. Ibrahim, and Qi-Man Shao. J Multivar Anal. 2009 October 1; 100(9): 2018–2030. This is available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2744117/ Your spline functions probably violate at least condition C2 of their Theorem 1. You'd probably see your mistake more clearly if, instead of -stcox- you fit -stpm- by Patrick Royston, or its extension,-stpm2- by Paul Lambert, both available at SSC. These commands use splines in the the time variable to fit the underlying survival functions. Your own functions would be collinear with these. You _can_ fit functions of time in -stcox- but only as interactions with other predictors, not as main effects. Steve On Fri, Jan 4, 2013 at 10:42 AM, Marc Peters <marcpeters32c@gmail.com> wrote: > 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 * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/faqs/resources/statalist-faq/ * http://www.ats.ucla.edu/stat/stata/