[R] weight factor in somers2 function

[Frank E Harrell Jr]

sara.vallecillo at ctfc.es wrote:
> Hi!
> I’m trying to calculate de C index (concordance probability) through the
> somers2 function (library Hmisc). I’m interesting on including the
> sampling effort as a weight factor for the evaluation of model predictions
> with real data. I’ve some questions about that: first of all I’m not
> really sure if I can include sampling effort as a weight factor. Since the
> weight factor should be a numeric vector of observation (usually
> frequencies), I would expect that sampling effort could be a surrogate of
> the frequency count of the number of subjects (i.e. frequency of
> observation). However, when I use sampling effort as a weight factor, I
> get C index larger than one. I guess/know this is statistically wrong.
> Then, if these values were frequency of observation; what is working
> incorrectly? What should be the characteristics of the weight vector? Or
> what could be exactly included as weight factor?
> Thank you very much!

Send me the smallest artificial example you can construct and I'll work 
on it.

Frank


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