Dear Saiwing Yeung, You appear to be using orthogonal-polynomial contrasts (generated by contr.poly) for Seed, which suggests that Seed is either an ordered factor or that you've assigned these contrasts to it. Because Seed has 14 levels, you end up fitting an degree-13 polynomial. If Seed is indeed an ordered factor and you want to use contr.treatment instead then you could, e.g., set Loblolly$Seed <- as.factor(Loblolly$Seed). (If I'm right about Seed being an ordered factor, your solution worked because it changed Seed to a factor, not because it used non-numeric level names.)
I hope this helps, John > -----Original Message----- > From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On > Behalf Of Saiwing Yeung > Sent: March-21-09 5:02 PM > To: r-help@r-project.org > Subject: [R] factor with numeric names > > Hi all, > > I have a pretty basic question about categorical variables but I can't > seem to be able to find answer so I am hoping someone here can help. I > found that if the factor names are all in numbers, fitting the model > in lm would return labels that are not very recognizable. > > # Example: let's just assume that we want to fit this model > fit <- lm(height ~ age + Seed, data=Loblolly) > > # See the category names are all mangled up here > fit > > > Call: > lm(formula = height ~ age + Seed, data = Loblolly) > > Coefficients: > (Intercept) age Seed.L Seed.Q Seed.C > Seed^4 > -1.31240 2.59052 4.86941 0.87307 0.37894 > -0.46853 > Seed^5 Seed^6 Seed^7 Seed^8 Seed^9 > Seed^10 > 0.55237 0.39659 -0.06507 0.35074 -0.83442 > 0.42085 > Seed^11 Seed^12 Seed^13 > 0.53906 -0.29803 -0.77254 > > > > One possible solution I found is to rename the categorical variables > > seed.str <- paste("S", Loblolly$Seed, sep="") > seed.str <- factor(seed.str) > fit <- lm(height ~ age + seed.str, data=Loblolly) > fit > > > > Call: > lm(formula = height ~ age + seed.str, data = Loblolly) > > Coefficients: > (Intercept) age seed.strS303 seed.strS305 seed.strS307 > -0.4301 2.5905 0.8600 1.8683 -1.9183 > seed.strS309 seed.strS311 seed.strS315 seed.strS319 seed.strS321 > 0.5350 -1.5933 -0.8867 -0.3650 -2.0350 > seed.strS323 seed.strS325 seed.strS327 seed.strS329 seed.strS331 > 0.3067 -1.3233 -2.6400 -2.9333 -2.2267 > > > Now it is actually possible to see which one is which, but is kind of > lame. Can someone point me to a more elegant solution? Thank you so > much. > > Saiwing Yeung > > ______________________________________________ > 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.
