[R] R vs SPSS contrasts

[Chuck Cleland]

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/articles/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
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-- 
Chuck Cleland, Ph.D.
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