On Jun 26, 2012, at 16:43 , r...@uni-potsdam.de wrote: > Hey, > today I wanted to use the shapiro.test() on data containing 3 numerical > values per group. > It is the first time that an NA was given back for some of the groups. > In the follwing an example of code and output is shown: > > >> shapiro.test(c(0.000637806, 0.00175561, 0.001196708)) > > Shapiro-Wilk normality test > > data: c(0.000637806, 0.00175561, 0.001196708) > W = 1, p-value = NA > > I am not able to find the bug in our data, so I think there might be a > problem with the shapiro.test().
The clue is that > diff(sort(c(0.000637806, 0.00175561, 0.001196708))) [1] 0.000558902 0.000558902 which is either an extreme coincidence or a sign that your data are not independent samples from a continuous distribution. Since the normal quantiles are also equidistant, you get a correlation of W=1 in the QQ-plot, and apparently this triggers the NA p-value. I suppose returning p=1.0 would arguably be a better choice for this case, but it _is_ pretty extreme. -pd > > I use the following technical background: > > platform x86_64-pc-linux-gnu > arch x86_64 > os linux-gnu > system x86_64, linux-gnu > status > major 2 > minor 14.1 > year 2011 > month 12 > day 22 > svn rev 57956 > language R > version.string R version 2.14.1 (2011-12-22) > > > Thanks, > Judith > > ______________________________________________ > 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. -- Peter Dalgaard, Professor, Center for Statistics, Copenhagen Business School Solbjerg Plads 3, 2000 Frederiksberg, Denmark Phone: (+45)38153501 Email: pd....@cbs.dk Priv: pda...@gmail.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.
