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 > <[EMAIL PROTECTED]> wrote: >> The formula should be (diag(n) - 1/n)[, -n] >> >> On Sun, Oct 12, 2008 at 1:36 PM, Gabor Grothendieck >> <[EMAIL PROTECTED]> 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 >>> <[EMAIL PROTECTED]> 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) <[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. >>>> >>> >> > > ______________________________________________ > 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. -------------------------------------------------------------------- E-Mail: (Ted Harding) <[EMAIL PROTECTED]> Fax-to-email: +44 (0)870 094 0861 Date: 12-Oct-08 Time: 21:04:48 ------------------------------ 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.