[R] upperbound of C index Conf.int. greater than 1

From: DAVID ARTETA GARCIA <darteta001_at_ikasle.ehu.es>
Date: Tue, 13 May 2008 14:16:12 +0200

R-users,

I am bootstrapping the C Index of a model created using lrm{Design} and boot{boot}, and I get that the upperbound of the confidence interval is greater than 1. Here is my code:

> library(HSAUR)
> data(plasma)

##fit model
> fit.design <- lrm (ESR ~ fibrinogen + globulin,data=plasma)
> fit.design$stats[6]

         C
0.8044872

##bootstrap C Index

> cindex <- function(formula,data,indices){

+ d=data[indices,]
+ fit<-lrm(formula,data = d)
+ return(fit$stats[[6]])
+ }

> results <- boot(data=w,statistic=cindex,R=500,formula = ESR ~
> fibrinogen + globulin)
> results

ORDINARY NONPARAMETRIC BOOTSTRAP Call:
boot(data = plasma, statistic = cindex, R = 500, formula = ESR ~ fibrinogen +

     globulin)

Bootstrap Statistics :

      original bias std. error
t1* 0.8044872 0.008834767 0.1574710
> boot.ci(results,type="basic")

BOOTSTRAP CONFIDENCE INTERVAL CALCULATIONS Based on 500 bootstrap replicates

CALL :
boot.ci(boot.out = results, type = "basic")

Intervals :
Level Basic
95% ( 0.6090, 1.1443 )
Calculations and Intervals on Original Scale

I see that the std.error is rather large and this might be the problem, but how can I explain this for publication purposes? Is such an interval acceptable?
Any help would be greatly appreciated

David



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