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) <[EMAIL PROTECTED]> > Fax-to-email: +44 (0)870 094 0861 > Date: 11-Oct-08 Time: 20:31:53 > ------------------------------ XFMail ------------------------------ > > ______________________________________________ > R-help@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@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.