[R] R vs SPSS contrasts
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. 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.
Re: [R] R vs SPSS contrasts
Don't know but perhaps you could just use each of: contr.helmert, contr.poly, contr.sum, contr.treatment, contr.SAS in turn on the R side until you get one that matches. Once you find out adding a contr.SPSS to R might be nice. On Sat, Oct 11, 2008 at 3:31 PM, Ted Harding <[EMAIL PROTECTED]> 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. > > > 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. > __ 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.
Re: [R] R vs SPSS contrasts
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$tension, contr=matrix(c(-1/3,2/3,-1/3,-1/3,-1/3,2/3), ncol=2)) ... attr(,"contrasts") [,1] [,2] L -0.333 -0.333 M 0.667 -0.333 H -0.333 0.667 Levels: L M H For a factor with 2 levels, like this: > C(warpbreaks$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.
Re: [R] R vs SPSS contrasts
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/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$tension, contr=matrix(c(-1/3,2/3,-1/3,-1/3,-1/3,2/3), > ncol=2)) > ... > attr(,"contrasts") >[,1] [,2] > L -0.333 -0.333 > M 0.667 -0.333 > H -0.333 0.667 > Levels: L M H > > For a factor with 2 levels, like this: > >> C(warpbreaks$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.
Re: [R] R vs SPSS contrasts
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/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$tension, contr=matrix(c(-1/3,2/3,-1/3,-1/3,-1/3,2/3), >> ncol=2)) >> ... >> attr(,"contrasts") >>[,1] [,2] >> L -0.333 -0.333 >> M 0.667 -0.333 >> H -0.333 0.667 >> Levels: L M H >> >> For a factor with 2 levels, like this: >> >>> C(warpbreaks$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.
Re: [R] R vs SPSS contrasts
I found this link: http://webs.edinboro.edu/EDocs/SPSS/SPSS%20Regression%20Models%2013.0.pdf 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/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$tension, contr=matrix(c(-1/3,2/3,-1/3,-1/3,-1/3,2/3), >>> ncol=2)) >>> ... >>> attr(,"contrasts") >>>[,1] [,2] >>> L -0.333 -0.333 >>> M 0.667 -0.333 >>> H -0.333 0.667 >>> Levels: L M H >>> >>> For a factor with 2 levels, like this: >>> C(warpbreaks$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.
Re: [R] R vs SPSS contrasts
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$tension, contr=matrix(c(-1/3,2/3,-1/3,-1/3,-1/3,2/3), ncol=2)) ... attr(,"contrasts") [,1] [,2] L -0.333 -0.333 M 0.667 -0.333 H -0.333 0.667 Levels: L M H For a factor with 2 levels, like this: > C(warpbreaks$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. ---
Re: [R] R vs SPSS contrasts
Gabor Grothendieck wrote: I found this link: http://webs.edinboro.edu/EDocs/SPSS/SPSS%20Regression%20Models%2013.0.pdf 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. Not sure about "have to"; if so, contr.SAS also needs extension since SAS does different things in different procedures (the help file does say so). Notice, BTW, that the cited reference defines contrast matrices inversely to R (it maps from group levels to parameters). -- O__ Peter Dalgaard Øster Farimagsgade 5, Entr.B c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K (*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918 ~~ - ([EMAIL PROTECTED]) FAX: (+45) 35327907 __ 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.
