Dear Dr Harrell, Thank you very much for your answer. Actually I also tried to found the C index by hand on these data using the mean probabilities and I found 0.968, as you just showed. I understand now why I had a slight difference with the outpout of lrm. I am thus convinced that this result is correct.
I read on the SAS help that the procedure logistic also proceed to some binning (BINWIDTH option) : http://support.sas.com/documentation/cdl/en/statug/63033/HTML/default/viewer.htm#statug_logistic_sect010.htm But I cannot explain why the difference between the two softwares is that huge, especially since the class probabilities are the same. Do you think it could be due to the fact that mean probabilities are computed differently ? Thank for your help and best regards, OC > Date: Thu, 24 Jan 2013 05:28:13 -0800 > From: f.harr...@vanderbilt.edu > To: r-help@r-project.org > Subject: Re: [R] Difference between R and SAS in Corcordance index in ordinal > logistic regression > > lrm does some binning to make the calculations faster. The exact calculation > is obtained by running > > f <- lrm(...) > rcorr.cens(predict(f), DA), which results in: > > C Index Dxy S.D. n missing > 0.96814404 0.93628809 0.03808336 32.00000000 0.00000000 > uncensored Relevant Pairs Concordant Uncertain > 32.00000000 722.00000000 699.00000000 0.00000000 > > I.e., C=.968 instead of .963. But this is even farther away than the value > from SAS you reported. > > If you don't believe the rcorr.cens result, create a tiny example and do the > calculations by hand. > Frank > > > blackscorpio81 wrote > > Dear R users, > > > > Please allow to me ask for your help. > > I am currently using Frank Harrell Jr package "rms" to model ordinal > > logistic regression with proportional odds. In order to assess model > > predictive ability, C concordance index is displayed and equals to 0.963. > > > > This is the code I used with the data attached > > data.csv <http://r.789695.n4.nabble.com/file/n4656409/data.csv> > > : > > > >>require(rms) > >>a<-read.csv2("/data.csv",row.names = 1,na.strings = c(""," "),dec=".") > >>lrm(DA~SJ+TJ,data=a) > > > > Logistic Regression Model > > > > lrm(formula = DA~SJ+TJ, data = a) > > > > Frequencies of Responses > > > > 1 2 3 4 > > 6 13 9 4 > > > > Model Likelihood > > Discrimination Rank Discrim. > > Ratio Test > > > > Indexes Indexes > > Obs 32 LR chi2 53.14 R2 > > > > 0.875 C 0.963 > > max |deriv| 6e-06 d.f. 2 g > > > > 8.690 Dxy 0.925 > > Pr(> chi2) <0.0001 gr > > > > 5942.469 gamma 0.960 > > > > > > gp 0.486 tau-a 0.673 > > > > > > Brier 0.022 > > > > Coef S.E. Wald Z Pr(>|Z|) > > y>=2 -0.6161 0.6715 -0.92 0.3589 > > y>=3 -6.5949 2.3750 -2.78 0.0055 > > y>=4 -16.2358 5.3737 -3.02 0.0025 > > SJ 1.4341 0.5180 2.77 0.0056 > > TJ 0.5312 0.2483 2.14 0.0324 > > > > I wanted to compare the results with SAS. I found the same slopes and > > intercept with opposite signs, which is normal since R models the > > probabilities P(Y>=k|X) whereas SAS models the probabilities P(Y<=k|X) > > (see pdf attached, page 2 , table "Association des probabilités prédites > > et des réponses observées"). > > SAS_Report_-_Logistic_Regression.pdf > > <http://r.789695.n4.nabble.com/file/n4656409/SAS_Report_-_Logistic_Regression.pdf> > > > > > > I chose the order for levels. > > > > I controlled that the corresponding probabilities P(Y=k|X) are the same > > with both softwares. But I can't understand why in SAS the C index drops > > from 0.963 down to 0.332. > > > > I read a lot of things about this and it seems to me that both softwares > > use slightly different technique to compute the C index ; it is > > nevertheless surprising to me to observe such a shift in the results. > > > > Does anyone have a clue on this ? > > Thank you very much for you help > > Blackscorpio > > > > > > ----- > Frank Harrell > Department of Biostatistics, Vanderbilt University > -- > View this message in context: > http://r.789695.n4.nabble.com/Difference-between-R-and-SAS-in-Corcordance-index-in-ordinal-logistic-regression-tp4656409p4656508.html > Sent from the R help mailing list archive at Nabble.com. > > ______________________________________________ > 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. [[alternative HTML version deleted]]
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