[Rd] R stat functions do not work as stated on the mannual ( PR#7419)

Liaw, Andy andy_liaw at merck.com
Tue Dec 14 12:41:26 CET 2004

> From: casadoj at ecc.es
> Dear R Developers:
> I have been playing with R, release 2.0.1 for a week now and 
> have detected =
> that all stat functions related to distribution probabilities 
> have the same=
>  problem:
> 1.- According to the manual the log.p parameter is always the 
> last one.
> 2.- When you use the software, the last parameter seems to be 
> lower.tail
> Example:
> > pt (1.1, 5)
> [1] 0.8392746
> > pt (1.1, 5, F)
> [1] 0.8392746
> > pt (1.1, 5, F, T)
> [1] 0.8392746
> >=0D
> On this example, I have used the Student T distribution. The 
> result of this=
>  example has been tested with the stat calculator at 
1.- This first line shows that when this function has two arguments, the lo=
g.p default value is false, and the lower.tail is true.
2.- The second line shows that when this function has 3 arguments, the last=
 one is the log.p argument and the lower.tail is taken by default to true.
3.- The third line shows that when this function has 4 arguments, the third=
 one keeps on being the log.p argument and the lower.tail is the last one.
4.- Acording to the mannual, the lower.tail should be the third argument an=
d not the last one.

Which manual did you read?  ?pt has:

pt(q, df, ncp=0, lower.tail = TRUE, log.p = FALSE)

So none of the calls you showed actually passed any thing for log.p to pt():
It's the 5th argument.  While it seems to be a convention that log.p is the
last argument, the actual position can change from distribution to
distribution as the number of parameters can differ.


Best regards,
Jos=E9 Luis Casado Mart=EDnez
European Computing Consultants
C/ Hermanos Garc=EDa Noblejas, N=BA 39, 5=AA, N 1
28037 Madrid
Telf.: 34-91-406 19 15. Fax: 34-91-406 19 16
Movil: 34-607-750 316

The last line shows that the sum of probabilities of 1.1 using 5 degrees of=
 freedom adding the lower tail (pt (1.1, 5, F, T)) and not adding the lower=
 tail ( pt (1.1, 5, F, F)) is 1. This is not the case with log (p).

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