[R] weight factor in somers2 function

[sara.vallecillo at ctfc.es]

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!