[R] wilcoxon-pratt signed rank test in R - drug-effiacy

[Peter Dalgaard]

"Niels Steen Krogh" <nielssteenkrogh at hotmail.com> writes:

> Hi.
> I'm going to introduce the R-package for a group of medical doctors
                             ^^^^^^^^^

Nitpick: R is a "language and environment (for statististical
computation and graphics)". An R-package is something that you load
into R, as in the contrib sections on CRAN.

> later this week and is a little confused about there use of a test
> named "willcoxon-pratt"  for testing if the clinical and biochemical
> markers has decreased significantly after the use of some drugs for a
> group of patients.
> 
> Looking into the R-functions I would in R recommand using a
> matched-pairs Wilcoxon test with a formula like:
> wilcox.test(pre,post,alternative='greater',paired=T)
> 
> Looking deeper into the writings of Pratt I found some 1964-stuff
> "Pratt JW. Remarks on zeros and ties in the Wilcoxon signed rank
> procedures.  J.Americ.Statistical Assoc. 1959; 54: 655-67. "
> 
> Do'es any of you know what is the wilcoxon-pratt test compared with
> the formula described above and how it should be implemented in R.

wilcox.test does the original Wilcoxon procedure, discarding any tied
pairs (zero difference). The Pratt procedure (rank all differences
first, *then* discard tied pairs) is not immediately available, but
would be fairly easy to implement, at least for the large-sample case.
I wouldn't be surprised if the exact procedure could be performed
using Torsten's ExactRankTests package (perm.test or wilcox.exact),
but I haven't gone deeply into it.

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907