Hello everyone. This is a question regarding generation of the concordance index (c index) in R using the function rcorr.cens. In particular about interpretation of its direction and form of the 'predictor'. One of the arguments is a "numeric predictor variable" ( presumably this is just a *single* predictor variable). Say this variable takes numeric values.... Am I correct in thinking that if the c index is > 0.5 (with Somers D positive) then this tells us that the higher the numeric values of the 'predictor', the greater the survival probability and similarly if the c index is <0.5 (with Somers D negative) then this tells us that the higher the numeric values of the 'predictor' the lower the survival probability ? The c index estimates the "probability of concordance between predicted and observed responses"....Harrel et al (1996) says "in predicting time until death, concordance is calculated by considering all possible pairs of patients, at least one of whom has died. If the *predicted* survival time (probability) is larger for the patient who (actually) lived longer, the predictions for that pair are said to be concordant with the (actual) outcomes. ". I have read that "the c index is defined by the proportion of all usable patients in which the predictions and outcomes are concordant". Now, secondly, I'd like to ask what form the predictor can take. Presumably if the predictor was a continuous-type variable e.g. 'age' then predicted survival probability (calculated internally via Cox regression?) would be compared with actual survival time for each specific age to get the c index? Now, if the predictor was an *ordinal categorical variable* where 1=worst group and 5=best group - I presume that the c index would be calculated similarly but this time there would be many ties in the predictor (as regards predicted survival probability) - hence if I wanted to count all ties in such a case I would keep the default argument outx=FALSE?
Does anyone have a clear reference which gives the formula used to generate the concordance index (with worked examples)? Many thanks for your help on these interpretations Kind Regards, Kim ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
