They will not be the same. The problem is that the `lower.tail' argument is not vectorized. Therefore, it is always set equal to the first element of `Resp', which in your example is FALSE.
If you want to obtain same results, this will do the trick: ans1 <- ifelse(Resp, log(pnorm(Xb)), log(1 - pnorm(Xb))) ans2 <- apply(cbind(Xb, Resp), 1, function(x) pnorm(x[1], lower.tail=x[2], log.p=TRUE)) all.equal(ans1, as.numeric(ans2)) Ravi. ____________________________________________________________________ Ravi Varadhan, Ph.D. Assistant Professor, Division of Geriatric Medicine and Gerontology School of Medicine Johns Hopkins University Ph. (410) 502-2619 email: rvarad...@jhmi.edu ----- Original Message ----- From: Ken Knoblauch <ken.knobla...@inserm.fr> Date: Tuesday, December 8, 2009 10:37 am Subject: [Rd] lower.tail option in pnorm To: r-devel@r-project.org > Hi, > > I would have thought that these two constructions would > produce the same result but they do not. > > Resp <- rbinom(10, 1, 0.5) > Stim <- rep(0:1, 5) > mm <- model.matrix(~ Stim) > Xb <- mm %*% c(0, 1) > ifelse(Resp, log(pnorm(Xb)), log(1 - pnorm(Xb))) > pnorm(as.vector(Xb), lower.tail = Resp, log.p = TRUE) > > > ifelse(Resp, log(pnorm(Xb)), log(1 - pnorm(Xb))) > [1] -0.6931472 -1.8410216 -0.6931472 -0.1727538 -0.6931472 > [6] -0.1727538 -0.6931472 -1.8410216 -0.6931472 -1.8410216 > > pnorm(as.vector(Xb), lower.tail = Resp, log.p = TRUE) > [1] -0.6931472 -1.8410216 -0.6931472 -1.8410216 -0.6931472 > [6] -1.8410216 -0.6931472 -1.8410216 -0.6931472 -1.8410216 > > If I have missed something obvious, I would be grateful > to have it pointed out. > > > sessionInfo() > R version 2.10.1 beta (2009-12-04 r50668) > i386-apple-darwin9.8.0 > > locale: > [1] en_US.UTF-8/en_US.UTF-8/C/C/en_US.UTF-8/en_US.UTF-8 > > attached base packages: > [1] stats graphics grDevices utils datasets methods > [7] base > > loaded via a namespace (and not attached): > [1] tools_2.10.1 > > Thanks for any enlightenment. > > best, > > Ken > > -- > Ken Knoblauch > Inserm U846 > Stem-cell and Brain Research Institute > Department of Integrative Neurosciences > 18 avenue du Doyen Lépine > 69500 Bron > France > tel: +33 (0)4 72 91 34 77 > fax: +33 (0)4 72 91 34 61 > portable: +33 (0)6 84 10 64 10 > > > ______________________________________________ > R-devel@r-project.org mailing list > ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel