Unexplained behavior of level names when using ordered factors in lm?
?ordered ?C ?contr.poly If you don't know what polynomial contrasts are, consult any good linear models text. MASS has a good, though a bit terse, section on this. -- Bert
On Fri, Dec 2, 2011 at 6:51 AM, Tal Galili <tal.galili at gmail.com> wrote:
Hello dear all, I am unable to understand why when I run the following three lines: set.seed(4254)
a <- data.frame(y = rnorm(40), x=ordered(sample(1:5, 40, T))) summary(lm(y ~ x, a))
The output I get includes factor levels which are not relevant to what I am actually using: Call:
lm(formula = y ~ x, data = a) Residuals: ? ? Min ? ? ?1Q ?Median ? ? ?3Q ? ? Max -1.4096 -0.6400 -0.1244 ?0.5886 ?2.1891 Coefficients: ? ? ? ? ? ? Estimate Std. Error t value Pr(>|t|) (Intercept) -0.03276 ? ?0.15169 ?-0.216 ? ?0.830 x.L ? ? ? ? -0.28968 ? ?0.33866 ?-0.855 ? ?0.398 x.Q ? ? ? ? -0.38813 ? ?0.33851 ?-1.147 ? ?0.259 x.C ? ? ? ? -0.27183 ? ?0.34027 ?-0.799 ? ?0.430 x^4 ? ? ? ? ?0.25993 ? ?0.33935 ? 0.766 ? ?0.449 Residual standard error: 0.9564 on 35 degrees of freedom Multiple R-squared: 0.08571, Adjusted R-squared: -0.01878 F-statistic: 0.8202 on 4 and 35 DF, ?p-value: 0.5211
I am guessing that this is having something to do with the contrast matrix that is used, but this is not clear to me. Can anyone suggest a good read, or an explanation? Thanks. ----------------Contact Details:------------------------------------------------------- Contact me: Tal.Galili at gmail.com | ?972-52-7275845 Read me: www.talgalili.com (Hebrew) | www.biostatistics.co.il (Hebrew) | www.r-statistics.com (English) ---------------------------------------------------------------------------------------------- ? ? ? ?[[alternative HTML version deleted]]
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Bert Gunter Genentech Nonclinical Biostatistics Internal Contact Info: Phone: 467-7374 Website: http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-biostatistics/pdb-ncb-home.htm