Patrick Drechsler wrote: > Hi, > > I am performing a two-way ANOVA (2 factors with 4 and 5 levels, > respectively). If I'm interpreting the output of summary correctly, > then the interaction between both factors is significant: > > ,---- > | ## Two-way ANOVA with possible interaction: > | > model1 <- aov(log(y) ~ xForce*xVel, data=mydataset) > | > | > summary(model1) > | Df Sum Sq Mean Sq F value Pr(>F) > | xForce 3 16.640 5.547 19.0191 1.708e-11 *** > | xVel 4 96.391 24.098 82.6312 < 2.2e-16 *** > | xForce:xVel 12 10.037 0.836 2.8681 0.0008528 *** > | Residuals 371 108.194 0.292 > | --- > | Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1 > | 3 observations deleted due to missingness > `---- > > To see the interactions in detail I call summary.lm: > > ,---- > | > summary.lm(model1) > | > | Call: > | aov(formula = log(y) ~ xForce * xVel, data = mydataset) > | > | Residuals: > | Min 1Q Median 3Q Max > | -2.04830 -0.32420 -0.04653 0.34928 1.46755 > | > | Coefficients: > | Estimate Std. Error t value Pr(>|t|) > | (Intercept) -1.663406 0.027335 -60.853 < 2e-16 *** > | xForce.L 0.408977 0.054726 7.473 5.68e-13 *** > | xForce.Q 0.101240 0.054670 1.852 0.0648 . > | xForce.C -0.068068 0.054613 -1.246 0.2134 > | xVel.L 1.079042 0.061859 17.444 < 2e-16 *** > | xVel.Q 0.339802 0.061439 5.531 6.03e-08 *** > | xVel.C 0.015422 0.060751 0.254 0.7997 > | xVel^4 -0.044399 0.060430 -0.735 0.4630 > | xForce.L:xVel.L 0.622060 0.123966 5.018 8.12e-07 *** > | xForce.Q:xVel.L 0.034298 0.123718 0.277 0.7818 > | xForce.C:xVel.L -0.114776 0.123470 -0.930 0.3532 > | xForce.L:xVel.Q 0.309293 0.123057 2.513 0.0124 * > | xForce.Q:xVel.Q 0.054798 0.122879 0.446 0.6559 > | xForce.C:xVel.Q -0.144219 0.122700 -1.175 0.2406 > | xForce.L:xVel.C 0.110588 0.121565 0.910 0.3636 > | xForce.Q:xVel.C -0.001929 0.121502 -0.016 0.9873 > | xForce.C:xVel.C -0.039477 0.121438 -0.325 0.7453 > | xForce.L:xVel^4 0.090491 0.120870 0.749 0.4545 > | xForce.Q:xVel^4 -0.002762 0.120861 -0.023 0.9818 > | xForce.C:xVel^4 -0.028836 0.120852 -0.239 0.8115 > | --- > | Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1 > | > | Residual standard error: 0.54 on 371 degrees of freedom > | (3 observations deleted due to missingness) > | Multiple R-Squared: 0.5322, Adjusted R-squared: 0.5082 > | F-statistic: 22.21 on 19 and 371 DF, p-value: < 2.2e-16 > `---- > > I am wondering what the logic is behind the formatting of > rownames. What do the strings "L", "Q", "C" and "^4" mean?
The default contrast for ordered factors is contr.poly(), and "L", "Q", "C" and "^4" refer to linear, quadratic, cubic, and quartic, respectively. You might look at some plots if you have not already. For example: with(mydataset, interaction.plot(xForce, xVel, log(y))) library(lattice) bwplot(log(y) ~ xForce | xVel, data = mydataset) > Apologies in case I missed something obvious in the relevant > documentations/archives. > > Thanks for any pointers, > > Patrick > > PS: Here are some details about the dataset: > > ,---- > | > summary(mydataset) > | y xForce xVel > | Min. :0.03662 0.01:97 10 :79 > | 1st Qu.:0.10376 0.1 :98 50 :80 > | Median :0.16314 1 :98 100 :80 > | Mean :0.26592 2 :98 500 :80 > | 3rd Qu.:0.28077 NA's: 3 5000:72 > | Max. :2.39490 NA's: 3 > | NA's :3.00000 > | > | > str(mydataset) > | 'data.frame': 394 obs. of 3 variables: > | $ y : num 0.167 0.158 0.152 0.158 0.131 ... > | $ xForce: Ord.factor w/ 4 levels "0.01"<"0.1"<"1"<..: 1 2 3 4 1 2 3 4 1 2 > ... > | $ xVel : Ord.factor w/ 5 levels "10"<"50"<"100"<..: 3 3 3 3 1 1 1 1 2 2 > ... > `---- > > I am using > > platform i486-pc-linux-gnu > arch i486 > os linux-gnu > system i486, linux-gnu > status > major 2 > minor 4.1 > year 2006 > month 12 > day 18 > svn rev 40228 > language R > version.string R version 2.4.1 (2006-12-18) > > ______________________________________________ > R-help@stat.math.ethz.ch 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. 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@stat.math.ethz.ch 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.