R help - Apr 2005 - How to calculate the AUC in R

Hello R-listers,

I'm working in an experiment that try to determine the degree of 
infection of different clones of a fungus and, one of the measures we 
use to determine these degree is the counting of antibodies in the 
plasma at different dilutions, in this experiment the maximum number of 
dilutions was eleven. I already checked for differences on the maximum 
concentration of the antibodies in function of each clone of the fungus. 
However one measure of interest is the area under the curve (AUC) for 
the counting of antibodies in function of dilution. Unfortunately I 
don't know how to calculate the AUC. Someone can point me an example of 
this procedure or a package that implements this calculation?

Thanks for the help,
Jo?o Paulo Dubas.

Dubas, Jo?o Paulo wrote:
> Hello R-listers,
> 
> I'm working in an experiment that try to determine the degree of 
> infection of different clones of a fungus and, one of the measures we 
> use to determine these degree is the counting of antibodies in the 
> plasma at different dilutions, in this experiment the maximum number of 
> dilutions was eleven. I already checked for differences on the maximum 
> concentration of the antibodies in function of each clone of the fungus. 
> However one measure of interest is the area under the curve (AUC) for 
> the counting of antibodies in function of dilution. Unfortunately I 
> don't know how to calculate the AUC. Someone can point me an example of
> this procedure or a package that implements this calculation?
> 
> Thanks for the help,
> Jo?o Paulo Dubas.
> 
One of many ways:

trap.rule <- function(x,y) sum(diff(x)*(y[-1]+y[-length(y)]))/2


-- 
Frank E Harrell Jr   Professor and Chair           School of Medicine
                      Department of Biostatistics   Vanderbilt University

Quoting Frank E Harrell Jr:
> Dubas, Jo?o Paulo wrote:
>
>> Quoting Frank E Harrell Jr:
>>
>> Do you know any book where I can get more information about the
subject?
>>
>> Thanks for the help
>> Jo?o Paulo Dubas.
>
>
> No, other than an algebra or calculus book where numerical integration 
> is discussion.
Thanks to all listers that helped me in this question.
The function AUC on the ROC package can be used for the analysis of of
the relationship sensitivity x sensibility of a test, I couldn't adjust
that function to my purpose.