[R] R vs SPSS contrasts

[(Ted Harding)]

Very many thanks, Chuck and Gabor, for the comments and the
references to on-line explanations. It is beginning to become
clear!

Most grateful.
Ted.

On 12-Oct-08 18:03:53, Gabor Grothendieck wrote:
> I found this link:
> 
> http://webs.edinboro.edu/EDocs/SPSS/SPSS%20Regression%20Models%2013.0.pd
> f
> 
> which indicates that the contrast in SPSS that is used
> depends not only on the contrast selected but also on the
> reference category selected and the two can be chosen
> independently.  Thus one could have simple/first, simple/last,
> deviation/first, deviation/last, etc.  An R contr.SPSS function
> would have to specify both the deviation type and the
> first/last in order to handle all SPSS variations.
> 
> On Sun, Oct 12, 2008 at 1:48 PM, Gabor Grothendieck
> <ggrothendieck at gmail.com> wrote:
>> The formula should be (diag(n) - 1/n)[, -n]
>>
>> On Sun, Oct 12, 2008 at 1:36 PM, Gabor Grothendieck
>> <ggrothendieck at gmail.com> wrote:
>>> Looks like the contrast matrix for indicator is contr.SAS(n),
>>> for deviation is contr.sum(n) and for simple is:
>>>
>>> (diag(n) - 1/n)[, -1]
>>>
>>> That works at least for the n = 3 example in the link.
>>> Perhaps the others could be checked against SPSS
>>> for a variety of values of n to be sure.
>>>
>>> On Sun, Oct 12, 2008 at 12:32 PM, Chuck Cleland
>>> <ccleland at optonline.net> wrote:
>>>> On 10/11/2008 3:31 PM, Ted Harding wrote:
>>>>> Hi Folks,
>>>>>
>>>>> I'm comparing some output from R with output from SPSS.
>>>>> The coefficients of the independent variables (which are
>>>>> all factors, each at 2 levels) are identical.
>>>>>
>>>>> However, R's Intercept (using default contr.treatment)
>>>>> differs from SPSS's 'constant'. It seems that the contrasts
>>>>> were set in SPSS using
>>>>>
>>>>>   /CONTRAST (varname)=Simple(1)
>>>>>
>>>>> I can get R's Intercept to match SPSS's 'constant' if I use
>>>>> contr.sum in R.
>>>>>
>>>>> Can someone please confirm that that is a correct match for
>>>>> the SPSS "Simple(1)", with identical effect?
>>>>>
>>>>> And is there a convenient on-line reference where I can look
>>>>> up what SPSS's "/CONTRAST" statements exactly mean?
>>>>> I've done a lot of googling, withbout coming up with anything
>>>>> satisfactory.
>>>>>
>>>>> With thanks,
>>>>> Ted.
>>>>
>>>> Hi Ted:
>>>>  Here are two links with the same content giving a brief description
>>>>  of
>>>> SPSS simple contrasts:
>>>>
>>>> http://www.ats.ucla.edu/stat/spss/library/contrast.htm
>>>> http://support.spss.com/productsext/spss/documentation/statistics/art
>>>> icles/contrast.htm
>>>>
>>>>  These pages explain how simple contrasts differ from indicator
>>>> (contr.treatment) and deviation (contr.sum) contrasts.  For a factor
>>>> with 3 levels, I believe you can reproduce SPSS simple contrasts
>>>> (with
>>>> the first category as reference) like this:
>>>>
>>>>> C(warpbreaks$tension, contr=matrix(c(-1/3,2/3,-1/3,-1/3,-1/3,2/3),
>>>> ncol=2))
>>>> ...
>>>> attr(,"contrasts")
>>>>        [,1]       [,2]
>>>> L -0.3333333 -0.3333333
>>>> M  0.6666667 -0.3333333
>>>> H -0.3333333  0.6666667
>>>> Levels: L M H
>>>>
>>>>  For a factor with 2 levels, like this:
>>>>
>>>>> C(warpbreaks$wool, contr=matrix(c(-1/2,1/2), ncol=1))
>>>> ...
>>>> attr(,"contrasts")
>>>>  [,1]
>>>> A -0.5
>>>> B  0.5
>>>> Levels: A B
>>>>
>>>>  Your description of the effect of SPSS simple contrasts - intercept
>>>> coefficient of contr.sum and non-intercept coefficients of
>>>> contr.treatment - sounds accurate to me.
>>>>
>>>> hope this helps,
>>>>
>>>> Chuck
>>>>
>>>>> --------------------------------------------------------------------
>>>>> E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
>>>>> Fax-to-email: +44 (0)870 094 0861
>>>>> Date: 11-Oct-08                                       Time:
>>>>> 20:31:53
>>>>> ------------------------------ XFMail
>>>>> ------------------------------
>>>>>
>>>>> ______________________________________________
>>>>> R-help at 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.
>>>>
>>>> --
>>>> Chuck Cleland, Ph.D.
>>>> NDRI, Inc. (www.ndri.org)
>>>> 71 West 23rd Street, 8th floor
>>>> New York, NY 10010
>>>> tel: (212) 845-4495 (Tu, Th)
>>>> tel: (732) 512-0171 (M, W, F)
>>>> fax: (917) 438-0894
>>>>
>>>> ______________________________________________
>>>> R-help at 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.
>>>>
>>>
>>
> 
> ______________________________________________
> R-help at 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.

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E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
Fax-to-email: +44 (0)870 094 0861
Date: 12-Oct-08                                       Time: 21:04:48
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