lower.tail option in pnorm

From the help page:

     pnorm(q, mean = 0, sd = 1, lower.tail = TRUE, log.p = FALSE)

     x,q: vector of quantiles.

lower.tail: logical; if TRUE (default), probabilities are P[X <= x],
          otherwise, P[X > x].

Note that lower.tail is not said to be a vector, and the first value is
taken (what it is is random in your example).


On Tue, 8 Dec 2009, Ken Knoblauch wrote:

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
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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
http://www.sbri.fr/members/kenneth-knoblauch.html

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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