From: Michael Grant <mwgrant2001_at_yahoo.com>

Date: Thu 13 Jul 2006 - 09:51:21 EST

R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Thu Jul 13 09:56:03 2006

Date: Thu 13 Jul 2006 - 09:51:21 EST

Matthew,

You may find the following documents useful if your venture into environmental statistics is serious.

First, the 92 EPA Addendum on GW statistics--links at http://www.epa.gov/correctiveaction/resource/guidance/sitechar/gwstats/gwstats.htm

The second is Helsel's book at the USGS

http://pubs.usgs.gov/twri/twri4a3/

Both documents have good discussions on normality tests for GW data including probability plot correlation coefficients and variations in the (x) plotting position--Blom, Cunane, etc.

Helsel is a good read 1.) his writing is so clear in his writing, 2.) he gets into nonparametric approaches in so many areas of GW stats, and 3.) the typography is nice--the book just a pleasant experience all around. Just be advised this is only the beginning...

Oh, yes. It ain't safe to just dabble with environmental (contaminant)data--it is too messy. Go whole hog or pass it up.

Best regards,

Michael Grant (works for the competition :O))

- Peter Dalgaard <p.dalgaard@biostat.ku.dk> wrote:

> <Matthew.Findley@ch2m.com> writes:

*>
**> > R Users:
**> >
**> > My question is probably more about elementary statistics than the
**> > mechanics of using R, but I've been dabbling in R (version 2.2.0) and
**> > used it recently to test some data .
**> >
**> > I have a relatively small set of observations (n = 12) of arsenic
**> > concentrations in background groundwater and wanted to test my
**> > assumption of normality. I used the Shapiro-Wilk test (by calling
**> > shapiro.test() in R) and I'm not sure how to interpret the output.
**> > Here's the input/output from the R console:
**> >
**> > >As = c(13, 17, 23, 9.5, 20, 15, 11, 17, 21, 14, 22, 13)
**> > >shapiro.test(As)
**> >
**> > Shapiro-Wilk normality test
**> >
**> > data: As
**> > W = 0.9513, p-value = 0.6555
**> >
**> > How do I interpret this? I understand, from poking around the internet,
**> > that the higher the W statistic the "more normal" the data.
**> >
**> > What is the null hypothesis - that the data is normally distributed?
**>
**> Yup.
**>
**> > What does the p-value tell me? 65.55% chance of what - getting
**> > W-statistic greater than or equal to 0.9513 (I picked this up from the
**> > Dalgaard book, Introductory Statistics with R, but its not really
**> > sinking in with respect to how it applies to a Shipiro Wilk test).?
**>
**> *Smaller* or equal - W=1.0 is the "perfect fit". The W statistic is
**> pretty much the Pearson correlation applied to the curve drawn by
**> qqnorm(). (The exact definition of what goes on the x axis differs
**> slightly, I believe.)
**>
**> A low p-value would indicate that the W is too extreme to be explained
**> by chance variation - i.e. evidence against normal distribution.
**> In the present case you have no evidence against normal distribution
**> (beware that this is not evidence _for_ normality).
**>
**> (Personally, I'm not too happy about these normality tests. They tend
**> to lack power in small samples and in large samples they often reject
**> distributions which are perfectly adequate for normal-theory
**> analysis. Learning to evaluate a QQ plot seems a better idea.)
**>
**>
**> > The method description - retrieved using ?shapiro.test() - is a bit
**> > light on details.
**>
**> There are references therein, though...
**>
**> --
**> O__ ---- Peter Dalgaard ุster Farimagsgade 5, Entr.B
**> c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K
**> (*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918
**> ~~~~~~~~~~ - (p.dalgaard@biostat.ku.dk) FAX: (+45) 35327907
**>
**> ______________________________________________
**> R-help@stat.math.ethz.ch mailing list
**> https://stat.ethz.ch/mailman/listinfo/r-help
**> PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
*

>

R-help@stat.math.ethz.ch mailing list

https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Thu Jul 13 09:56:03 2006

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