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Re: st: RE: Interpretation of tvc in stcox


From   Jennifer Evans <evansjenniferl@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: RE: Interpretation of tvc in stcox
Date   Thu, 8 May 2008 11:32:56 -0700 (PDT)

Hi Dianna, 

Thank you for your interest.  To answer your questions, yes, the texp(_t) is how the variables
change with time. It is the default so did not need to be included in the stcox statement.  

My data is set up so that there are multiple records per individual, with different rows for each
visit.  Using the 'id' option in the stset statement, Stata will know how to identify records
within an individual, ie. 360 subjects, 1577 records: 

No. of subjects =          360                     Number of obs   =      1577

Behavioral variables were measured at each visit (time varying), and were specified with the tvc
option. 


Best, 
Jennifer


--- Dianna Magliano <dmagliano@idi.org.au> wrote:

> Hi Jennifer,
> 
> I wanted to ask:
> 
> You have specified which variables are time varying but havent specified how they change. 
> ie. Is the texp(_t) bit optional?
> 
> You also mention that you split your data into multiple record.s Do you have variables measured
> at these different time points.
> 
> How does stata know when to apply the updated covariate which has changed over time?
> 
> Dianna
> 
> Dr Dianna Magliano, BAppSci(Hons), MPH, PhD
> Senior Epidemiologist
> International Diabetes Institute
> 250 Kooyong Road
> Caulfield Victoria 3162
> Ph: 03 9258 5931
> 0425 706 637
> 
> 
> 
> 
> 
> Dianna Magliano BAppSci(Hon) MPH PhD | Epidemiologist
> International Diabetes Institute | 250 Kooyong Road | Caulfield | VIC 3162. Australia. 
> p +61 (0)3 9258 5931 f +61 (0)3 9258 5090 e dmagliano@idi.org.au | www.diabetes.com.au 
> 
> 
> The International Diabetes Institute and the Baker Heart Research Institute are merging during
> 2008
> 
> 
>  
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> 
> -----Original Message-----
> 
> From: owner-statalist@hsphsun2.harvard.edu on behalf of Jennifer Evans
> Sent: Thu 08/05/2008 7:46 AM
> To: statalist@hsphsun2.harvard.edu
> Subject: st: Interpretation of tvc in stcox
>  
> Dear statalist,
>    
>   I have a dataset with multiple records per person with time varying covariates. I am running
> survival analysis with the tvc option in stcox and have found 2 variables with significant time
> interactions.  My questions are:
> (1) Am I setting up the stcox statement correctly (see code below)
> (2) Regarding the interpretation- Is it correct to say that the variable var1 has a hazard of
> 0.35
> with the relative hazard then decreasing by a factor of 1.00 with each unit of _t?   
> 
> Any comments or suggestions would be appreciated! 
> 
> Best regards,
> Jennifer Evans
> 
> 
> . xi: stcox yrsinj_b age_b gender_b var1 var2 var3 var4,  nolog noshow tvc(var1 var3) 
> 
> Cox regression -- Breslow method for ties
> 
> No. of subjects =          360                     Number of obs   =      1577
> No. of failures =          104
> Time at risk    =       226907
>                                                    LR chi2(16)     =    117.25
> Log likelihood  =   -467.29121                     Prob > chi2     =    0.0000
> 
> ------------------------------------------------------------------------------
>           _t | Haz. Ratio   Std. Err.      z    P>|z|     [95% Conf. Interval]
> -------------+----------------------------------------------------------------
> rh           |
>     yrsinj_b |   1.033031    .029971     1.12   0.263     .9759283    1.093476
>        age_b |   .9916956   .0319112    -0.26   0.796     .9310822    1.056255
>     gender_b |   1.114194   .2912036     0.41   0.679     .6675631    1.859641
>         var1 |   .3489511   .1266303    -2.90   0.004     .1713457    .7106501
>         var2 |   .3903011   .1237031    -2.97   0.003     .2097093    .7264101
>         var3 |   .2174367   .0915801    -3.62   0.000     .0952409     .496412
>         var4 |   1.562432   .3349837     2.08   0.037     1.026371    2.378471
> -------------+----------------------------------------------------------------
> t            |
>         var1 |   1.001592    .000593     2.69   0.007      1.00043    1.002755
>         var3 |   1.001354   .0005982     2.26   0.024     1.000182    1.002527
> ------------------------------------------------------------------------------
> 
> 
> 
> 
> 
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