Re: [R] how to specify a particular contrast

From: Ross Darnell <r.darnell_at_uq.edu.au>
Date: Fri, 18 Jan 2008 12:45:08 +1000

I don't quite follow what you are trying to do but the second contrast has a few interpretations with the same meaning in your case

  1. are the 2-1 and 3-2 differences equal
  2. lack of fit of a linear trend
  3. is there a quadratic response

If you declare your factor to be "ordered" then the default contrasts will be poly()nomials.

Ross Darnell

-----Original Message-----
From: r-help-bounces_at_r-project.org [mailto:r-help-bounces_at_r-project.org] On Behalf Of array chip
Sent: Friday, 18 January 2008 10:22 AM
To: r-help_at_stat.math.ethz.ch
Subject: [R] how to specify a particular contrast

Hi, I am running a simple one-way ANOVA with an independent factot variable "treat" (3 levels: a, b and c) and a response variable "y". I want to test a linear relationship of the response among the 3 levels of the variable "treat" (ordered a->b->c). I used glht() from multcomp package. Later I found out I need to exclude the situation where the response at the 3 levels of "treat" are equal. I can do separate contrasts to test them separately:

obj<-aov(y~treat,data=dat)
  ### testing a=b=c
summary(glht(obj, linfct= mcp
(treat=c('a-b=0','a-c=0','b-c=0'))),test=Ftest())
  ### testing linear relationship among a,b and c summary(glht(obj, linfct= mcp
(treat=c('a+c-2*b=0'))),test=Ftest())

Is there anyway to build one contrast that tests both at the same time, i.e. just generate one single p value. Because the ultimate purpose was to test the linear relationship among the 3 levels of the variable "treat". Or I am asking something that is non-sensible to do?

Thanks

John Zhang  




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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. Received on Fri 18 Jan 2008 - 02:52:25 GMT

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