(x1 + x2 + x3) ^2 will give you the main effects and the interactions. So this will shorten your equation to
ft <- lm(y ~ (x1 + x2 + x3) ^2 + I(x1 * x1)+ I(x2 * x2) + I(x3 * x3), mydata) HTH, Thierry ------------------------------------------------------------------------ ---- ir. Thierry Onkelinx Instituut voor natuur- en bosonderzoek / Research Institute for Nature and Forest Cel biometrie, methodologie en kwaliteitszorg / Section biometrics, methodology and quality assurance Gaverstraat 4 9500 Geraardsbergen Belgium tel. + 32 54/436 185 [EMAIL PROTECTED] www.inbo.be To call in the statistician after the experiment is done may be no more than asking him to perform a post-mortem examination: he may be able to say what the experiment died of. ~ Sir Ronald Aylmer Fisher The plural of anecdote is not data. ~ Roger Brinner The combination of some data and an aching desire for an answer does not ensure that a reasonable answer can be extracted from a given body of data. ~ John Tukey -----Oorspronkelijk bericht----- Van: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] Namens Jinsong Zhao Verzonden: zondag 27 juli 2008 9:39 Aan: r-help@r-project.org Onderwerp: [R] A easy way to write formula Hi I have a data frame, including x1, x2, x3, and y. I use lm() to fit second-order linear model, like the following: ft <- lm(y ~ x1 + x2 + x3 + I(x1 * x1) + I(x1 * x2) + I(x1 * x3) + I(x2 * x2) + I(x2 * x3) + I(x3 * x3), mydata) if the independent variable number is large, the formula will be very long. Is there a easy way to write formula like the above one? I have read the R introduction, however, I don't find a easy way. Any hints will be appreciated. Thanks, Jinsong ______________________________________________ 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.
