The way to understand this is to look at the output of model.matrix: model.matrix(fo, fr)
for each formula you tried. If your data is large you will have to use a subset not to be overwhelmed with output. On Fri, Mar 5, 2010 at 9:08 AM, blue sky <bluesky...@gmail.com> wrote: > Suppose, 'fr' is data.frame with columns 'Y', 'A' and 'B'. 'A' has levels 'Aa' > 'Ab' and 'Ac', and 'B' has levels 'Ba', 'Bb', 'Bc' and 'Bd'. 'Y' > columns are numbers. > > I tried the following three sets of commands. I understand that A*B is > equivalent to A+B+A:B. However, A:B in A+B+A:B is different from A:B > just by itself (see the 3rd and 4th set of commands). Would you please > help me understand why the meanings of A:B are different in different > contexts? > > I also see the coefficient of AAc:BBd is NA (the last set of > commands). I'm wondering why this coefficient is not removed from the > 'coefficients' vector. Since lm(Y~A) has coefficients for (intercept), > Ab, Ac (there are no NA's), I think that it is reasonable to make sure > that there are no NA's when there are interaction terms, namely, A:B > in this case. > > Thank you for answering my questions! > >> alm=lm(Y ~ A*B,fr) >> alm$coefficients > (Intercept) AAb AAc BBb BBc BBd > -3.548176 -2.086586 7.003857 4.367800 11.887356 -3.470840 > AAb:BBb AAc:BBb AAb:BBc AAc:BBc AAb:BBd AAc:BBd > 5.160865 -11.858425 -12.853116 -20.289611 6.727401 -2.327173 >> >> alm=lm(Y ~ A + B + A:B,fr) >> alm$coefficients > (Intercept) AAb AAc BBb BBc BBd > -3.548176 -2.086586 7.003857 4.367800 11.887356 -3.470840 > AAb:BBb AAc:BBb AAb:BBc AAc:BBc AAb:BBd AAc:BBd > 5.160865 -11.858425 -12.853116 -20.289611 6.727401 -2.327173 >> >> alm=lm(Y ~ A:B - 1,fr) >> alm$coefficients > AAa:BBa AAb:BBa AAc:BBa AAa:BBb AAb:BBb AAc:BBb AAa:BBc > -3.5481765 -5.6347625 3.4556808 0.8196231 3.8939016 -4.0349449 8.3391795 > AAb:BBc AAc:BBc AAa:BBd AAb:BBd AAc:BBd > -6.6005222 -4.9465744 -7.0190168 -2.3782017 -2.3423322 >> >> alm=lm(Y ~ A:B,fr) >> alm$coefficients > (Intercept) AAa:BBa AAb:BBa AAc:BBa AAa:BBb AAb:BBb > -2.34233221 -1.20584424 -3.29243033 5.79801305 3.16195534 6.23623377 > AAc:BBb AAa:BBc AAb:BBc AAc:BBc AAa:BBd AAb:BBd > -1.69261273 10.68151168 -4.25819000 -2.60424217 -4.67668454 -0.03586951 > AAc:BBd > NA > > ______________________________________________ > 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.