[R] Dunn's post hoc test

[Robert Baer]

I think Martin told you the basic approach to the indexing:

averank<-sort(sample(1:100,25,replace=TRUE))
averank[-1] - averank[-length(averank)]
 [1]  1  1  6  3  4 14  1  1  8  1  2  6  5  4 10  0  3  2  1 11  1  1  2  0
averank
 [1]  4  5  6 12 15 19 33 34 35 43 44 46 52 57 61 71 71 74 76 77 88 89 90 92
92


----- Original Message ----- 
From: "IAIN GALLAGHER" <iaingallagher at btopenworld.com>
To: "Martin Henry H. Stevens" <HStevens at MUOhio.edu>
Cc: <r-help at stat.math.ethz.ch>
Sent: Monday, October 17, 2005 2:29 PM
Subject: Re: [R] Dunn's post hoc test


> Thanks for your reply Hank. It's not really what I'm
> after (though it's good to know).
>
> For the test ( as described in Statistics for the
> Biosciences by W. Gardiner. Prentice Hall, 1997) I
> have to rank my groups, calculate the average rank,
> then subtratc each average rank from every other. Any
> value greater than the test statistic is significant.
>
> eg average rank difference table:
>
>    2     5     8    9
> ---|------------------ 
> 2  -     3     6    7
>    |
> 5  -     -     3    4
>    |
> 8  -     -     -    1
>    |
> 9  -     -     -    -
>    |
>
> I can't get my head around writing an algorithm for
> this if I have a vector of average ranks eg averank<-
> c(2,5,8,9).
>
> I know I can address the vector by index and that this
> is probably the correct route but I can't get the
> indexing algorithm right!
>
> I'm sure someone will point out somethng simple and
> I'll kick myself but the help would be appreciated.
>
> Thanks again.
>
> Iain Gallagher
>
> --- "Martin Henry H. Stevens" <HStevens at MUOhio.edu>
> wrote:
>
> > I don't know Dunn's rank test, but the following
> > substracts each of
> > the sums of averanks from the next rank.
> >
> > cumsum(averank)[-length(averank)] - averank[-1]
> >
> > Hank
> >
> > On Oct 17, 2005, at 4:30 AM, Iain Gallagher wrote:
> >
> > > Hi Everyone.
> > >
> > > I am rather new to R and I've been trying to
> > implement a function to
> > > carry out the above test. For a couple of days now
> > I've been stuck on
> > > how to generate average rank differences.
> > >
> > > Say I have a vector of average ranks:
> > >
> > > averank<- c(2,5,9,12)
> > >
> > > I would like to subtract averank[1] from
> > averank[2], averank[1] and
> > > averank[2] from averank[3] and averank[1],
> > averank[2] and averank[3]
> > > from averank[4] etc (I know the syntax is wrong
> > here... it's just for
> > > illustration) but I can't work out how to do it.
> > >
> > > Ideally I would like to generate an array showing
> > the differences
> > > between the average ranks so I could tell at a
> > glance which ones were
> > > greater than my critical value
> > >
> > > I've been looking at loops etc but it's a little
> > beyond me at the
> > > moment. Thanks for any suggestions.
> > >
> > > Iain Gallagher
> > > IIIR
> > > Edinburgh University
> > >
> > > ______________________________________________
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> > > PLEASE do read the posting guide!
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> > >
> >
> > Dr. Martin Henry H. Stevens, Assistant Professor
> > 338 Pearson Hall
> > Botany Department
> > Miami University
> > Oxford, OH 45056
> >
> > Office: (513) 529-4206
> > Lab: (513) 529-4262
> > FAX: (513) 529-4243
> > http://www.cas.muohio.edu/~stevenmh/
> > http://www.muohio.edu/ecology/
> > http://www.muohio.edu/botany/
> > "E Pluribus Unum"
> >
> >
>
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