[R] What is the effect of how.many in the "contrast" function

Prof Brian Ripley ripley at stats.ox.ac.uk
Fri Feb 14 07:41:24 CET 2014

There is no contrast() function in R itself, but this seems to be about 

On 14/02/2014 04:35, jimj wrote:
> I have been asked to see if there is a linear trend in 3 groups of data (5
> points each) by using ANOVA and linear contrasts. The 3 groups represent
> data collected in 2010,2011 and 2012. I want to use R for this procedure and
> I have tried both of the following:
>      contrasts(data$groups, how.many=1) <- contr.poly(3)
> contrasts(data$groups)  <- contr.poly(3)
> Both ways seem to work fine but give slightly different answers in terms of
> their p-values. I have no idea which is correct and it is really tricky to
> find help for this on the web. I would like help figuring out what is the
> reasoning behind the different answers. I'm not sure if it has something to
> do with partitioning sums of squares or whatnot. Thanks so much for your
> help.

You should always read the help page, which does explain this.  Maybe 
you need the background of Chapter 6 of MASS (the book)?

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Please do.  Had you shown an example we could probably have explained 
this to you.

Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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