[R] logistic regression lrm() output

[Frank Harrell]

Why is a one unit change in x an interesting range for the purpose of
estimating an odds ratio?

The default in summary() is the inter-quartile-range odds ratio as clearly
stated in the rms documentation.
Frank

array chip wrote:
> 
> Hi, I am trying to run a simple logistic regression using lrm() to
> calculate a 
> odds ratio. I found a confusing output when I use summary() on the fit
> object 
> which gave some OR that is totally different from simply taking 
> exp(coefficient), see below:
> 
>> dat<-read.table("dat.txt",sep='\t',header=T,row.names=NULL)
> 
>> d<-datadist(dat)
>> options(datadist='d')
>> library(rms)
>> (fit<-lrm(response~x,data=dat,x=T,y=T))
> 
> Logistic Regression Model
> lrm(formula = response ~ x, data = dat, x = T, y = T)
> 
>                       Model Likelihood     Discrimination    Rank Discrim.    
>                          Ratio Test            Indexes          Indexes       
> 
> Obs           150    LR chi2      17.11    R2       0.191    C       0.763    
>  0            128    d.f.             1    g        1.209    Dxy     0.526    
>  1             22    Pr(> chi2) <0.0001    gr       3.350    gamma   0.528    
> max |deriv| 1e-11                          gp       0.129    tau-a   0.132    
>                                            Brier    0.111                     
> 
>           Coef    S.E.   Wald Z Pr(>|Z|)
> Intercept -5.0059 0.9813 -5.10  <0.0001 
> x          0.5647 0.1525  3.70  0.0002 
> 
> As you can see, the odds ratio for x is exp(0.5647)=1.75892.
> 
> But if I run the following using summary():
> 
>> summary(fit)
>              Effects              Response : response 
> 
>  Factor      Low    High   Diff.  Effect S.E. Lower 0.95 Upper 0.95
>  x           3.9003 6.2314 2.3311 1.32   0.36 0.62       2.01      
>   Odds Ratio 3.9003 6.2314 2.3311 3.73     NA 1.86       7.49
> 
> What are these output? none of the numbers is the odds ratio (1.75892)
> that I 
> calculated by using exp().
> 
> Can any explain?
> 
> Thanks
> 
> John
> 	[[alternative HTML version deleted]]
> 
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> 


-----
Frank Harrell
Department of Biostatistics, Vanderbilt University
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