R help - Nov 2006 - Logistic regression model (Urjent help needed)

I am using logistic regression model (lrm) of package Design.

Can some one please tell me how to calculate the average Area Under Curve
(AUC) for n-fold cross-validation

The help for lrm function says to do cross validation like this

f <- lrm( cy ~ x1 + x2, x=TRUE, y=TRUE)
val <- validate.lrm(f, method="cross", B=5)

Now I dont know what to do with variable "val" to get the average AUC
value.

thnx and regards,
nitin

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nitin jindal wrote:
> I am using logistic regression model (lrm) of package Design.
> 
> Can some one please tell me how to calculate the average Area Under Curve
> (AUC) for n-fold cross-validation
> 
> The help for lrm function says to do cross validation like this
> 
> f <- lrm( cy ~ x1 + x2, x=TRUE, y=TRUE)
> val <- validate.lrm(f, method="cross", B=5)
You can just say validate( . . . ) instead of validate.lrm
> 
> Now I dont know what to do with variable "val" to get the average
AUC value.
Just print it and look at "index.corrected" under the Dxy row. 
Somer's
D rank correlation is related to AUC (C) by Dxy=2(C-.5)

Note that the default method of 'boot' works much better than 
cross-validation for this purpose, and both methods assume that x1 and 
x2 were the only variables or transformations examined.

Frank
> 
> thnx and regards,
> nitin
> 
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> 
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-- 
Frank E Harrell Jr   Professor and Chair           School of Medicine
                      Department of Biostatistics   Vanderbilt University