Dear Matthew, The easiest way the get the estimates (and their standard error) for the different slopes it to reparametrise your model. Use resp ~ var1 : cat + 0 instead of resp ~ var1 * cat
HTH, Thierry ------------------------------------------------------------------------ ---- ir. Thierry Onkelinx Instituut voor natuur- en bosonderzoek team Biometrie & Kwaliteitszorg Gaverstraat 4 9500 Geraardsbergen Belgium Research Institute for Nature and Forest team Biometrics & Quality Assurance Gaverstraat 4 9500 Geraardsbergen Belgium tel. + 32 54/436 185 thierry.onkel...@inbo.be 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: r-help-boun...@r-project.org > [mailto:r-help-boun...@r-project.org] Namens Matthew Carroll > Verzonden: maandag 12 april 2010 11:16 > Aan: r-help@r-project.org > Onderwerp: [R] Interpreting factor*numeric interaction coefficients > > Dear all, > I am a relative novice with R, so please forgive any terrible > errors... > > I am working with a GLM that describes a response variable as > a function of a categorical variable with three levels and a > continuous variable. These two predictor variables are > believed to interact. > An example of such a model follows at the bottom of this > message, but here is a section of its summary table: > > Estimate Std. Error z value Pr(>|z|) > (Intercept) 1.220186 0.539475 2.262 0.0237 * > var1 0.028182 0.050850 0.554 0.5794 > cat2 -0.112454 0.781137 -0.144 0.8855 > cat3 0.339589 0.672828 0.505 0.6138 > var1:cat2 0.007091 0.068072 0.104 0.9170 > var1:cat3 -0.027248 0.064468 -0.423 0.6725 > > I am having trouble interpreting this output. > I think I understand that: > > # the 'var1' value refers to the slope of the relationship > within the first factor level > > # the 'cat2' and 'cat3' values refer to the difference in > intercept from 'cat1' > > # the interaction terms describe the difference in slope > between the relationship in 'cat1' and that in 'cat2' and > 'cat3' respectively > > Therefore, if I wanted a single value to describe the slope > in either cat2 or cat3, I would sum the interaction value > with that of var1. > > However, if I wanted to report a standard error for the slope > in 'cat2', how would I go about doing this? Is the reported > standard error that for the overall slope for that factor > level, or is the actual standard error a function of the > standard error of var1 and that of the interaction? > > Any help with this would be much appreciated, > > Matthew Carroll > > > ### example code > > resp <- rpois(30, 5) > cat <- factor(rep(c(1:3), 10)) > var1 <- rnorm(30, 10, 3) > > mod <- glm(resp ~ var1 * cat, family="poisson") > summary(mod) > > Call: > glm(formula = resp ~ var1 * cat, family = "poisson") > > Deviance Residuals: > Min 1Q Median 3Q Max > -1.80269 -0.54107 -0.06169 0.51819 1.58169 > > Coefficients: > Estimate Std. Error z value Pr(>|z|) > (Intercept) 1.220186 0.539475 2.262 0.0237 * > var1 0.028182 0.050850 0.554 0.5794 > cat2 -0.112454 0.781137 -0.144 0.8855 > cat3 0.339589 0.672828 0.505 0.6138 > var1:cat2 0.007091 0.068072 0.104 0.9170 > var1:cat3 -0.027248 0.064468 -0.423 0.6725 > --- > Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1 > > (Dispersion parameter for poisson family taken to be 1) > > Null deviance: 23.222 on 29 degrees of freedom Residual > deviance: > 22.192 on 24 degrees of freedom > AIC: 133.75 > > Number of Fisher Scoring iterations: 5 > > > > -- > Matthew Carroll > E-mail: mjc...@york.ac.uk > > ______________________________________________ > 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. > Druk dit bericht a.u.b. niet onnodig af. Please do not print this message unnecessarily. Dit bericht en eventuele bijlagen geven enkel de visie van de schrijver weer en binden het INBO onder geen enkel beding, zolang dit bericht niet bevestigd is door een geldig ondertekend document. The views expressed in this message and any annex are purely those of the writer and may not be regarded as stating an official position of INBO, as long as the message is not confirmed by a duly signed document. ______________________________________________ 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.
