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Re: st: Large standard error, Cox PH


From   Lee Savage <leemsavage@yahoo.co.uk>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   Re: st: Large standard error, Cox PH
Date   Sat, 28 Jul 2012 13:48:12 +0100 (BST)

The study is an analysis of government termination, estimated using a Cox proportional hazards model. The problem variable is 'Minority', this is a binary variable that indicates whether or not a government holds a parliamentary majority. The problem is that the standard error of the coefficient is extremely high. I have only seen this before when the coefficient was insignificant but in this case the coefficient is significant (as you can see below). Multicollinearity isn't a problem. I'm looking for advice on whether or not this is a problem or can I simply report the model and just state that the high SE of the 'Minority' variable means that it can't really be generalized?

Here is the printout. 

Iteration 0:   log pseudolikelihood = -40.288812
Iteration 1:   log pseudolikelihood = -28.304301
Iteration 2:   log pseudolikelihood = -26.968036
Iteration 3:   log pseudolikelihood = -26.902024
Iteration 4:   log pseudolikelihood = -26.901788
Refining estimates:
Iteration 0:   log pseudolikelihood = -26.901788

Cox regression -- Breslow method for ties

No. of subjects      =           19                Number of obs   =       347
No. of failures      =           19
Time at risk         =          347
                                                   Wald chi2(7)    =   1603.76
Log pseudolikelihood =   -26.901788                Prob > chi2     =    0.0000

                            Haz.     Robust
                            Ratio    SE        z        P>z      [95% Conf.Interval]
Minority               77.01     56.61    5.91    0.00      18.23   325.28
Ideology                0.84      0.20    -0.73    0.47      0.52    1.35
formdays               0.94      0.03    -2.16    0.03      0.90    0.99
nogovtpart~s         1.51      1.68     0.37    0.71      0.17    13.34
ciep12                  1.36      1.25     0.34    0.74      0.23    8.21
ConsNoCon          1.28      1.18     0.27    0.79      0.21    7.86
                        
tvc                        
Unemployment     0.99      0.01    -2.31    0.02      0.98    1.00
GDP                    1.00      0.00    2.24    0.03      1.00    1.00
Inflation                0.98      0.01    -4.38    0.00      0.97    0.99




__________________________


From: Steve Samuels <sjsamuels@gmail.com>
To: statalist@hsphsun2.harvard.edu 
Sent: Friday, 27 July 2012, 21:30
Subject: Re: st: Large standard error, Cox PH


To answer your questions, we'd need more detail.  Describe the study and the problem variable in particular.
As the FAQ request, "Say exactly what you typed and exactly what Stata typed (or did) in response". 

Steve
sjsamuels@gmail.com




On Jul 27, 2012, at 2:20 PM, Lee Savage wrote:

I have estimated a Cox PH model using a small sample (n=19, 347 months at risk). For one of my covariates I have found a large hazard ratio (77.01) with a correspondingly large standard error (56.61). I have seen this before but every time the covariate was insignificant, in the current model the covariate is significant (p=.001). I have tested the covariates for collinearity and everything looks fine. I think the probable cause is the small sample size.


So my question is: is this a problem for my model overall model? My inclination is to report the model as it is and just state that the significant effect for the covariate in question should be treated with extreme caution, perhaps even ignored.

I'd appreciate any advice on this.

Thanks.

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