Dear Tomas, Write the model as
mreg01 = lm(enep1 ~ enpres * proximity1), data=a90) That is, it's not necessary to index a90 as a list since it's given as the data argument to lm, and doing so confuses the effect() function. Also, enpres*proximity1 will include both the enpres:proximity1 interaction and enpres + proximity1, which are marginal to the interaction. Next, you must quote the name of the term for which you want to compute effects, thus "enpres:proximity1" in the call to effect(). Finally, effect() doesn't compute what are usually termed marginal effects. If you want more information about what it does, see the references given in ?effect. I hope this helps, John ------------------------------------------------ John Fox Sen. William McMaster Prof. of Social Statistics Department of Sociology McMaster University Hamilton, Ontario, Canada http://socserv.mcmaster.ca/jfox/ On Thu, 31 Mar 2011 22:09:32 +0200 Tomii <dioge...@gmail.com> wrote: > Hello, > > I try to plot the marginal effect by using package "effects" (example of the > graph i want to get is in the attached picture). > All variables are continuous. > > Here is regression function, results and error effect function gives: > > > mreg01 = lm(a90$enep1 ~ a90$enpres + a90$proximity1 + (a90$enpres * > > a90$proximity1), data=a90)> summary(mreg01) > Call: > lm(formula = a90$enep1 ~ a90$enpres + a90$proximity1 + (a90$enpres * > a90$proximity1), data = a90) > > Residuals: > Min 1Q Median 3Q Max > -2.3173 -1.3349 -0.5713 0.8938 8.1084 > > Coefficients: > Estimate Std. Error t value Pr(>|t|) > (Intercept) 4.2273 0.3090 13.683 < 2e-16 *** > a90$enpres 0.4225 0.2319 1.822 0.072250 . > a90$proximity1 -3.8797 1.0984 -3.532 0.000696 *** > a90$enpres:a90$proximity1 0.8953 0.4101 2.183 0.032025 * > --- > Signif. codes: 0 *** 0.001 ** 0.01 * 0.05 . 0.1 1 > > Residual standard error: 2.029 on 78 degrees of freedom > Multiple R-squared: 0.2128, Adjusted R-squared: 0.1826 > F-statistic: 7.031 on 3 and 78 DF, p-value: 0.0003029 > > plot(effect(a90$enpres:a90$proximity1, mreg01))Warning messages:1: In > > a90$enpres:a90$proximity1 : > numerical expression has 82 elements: only the first used2: In > a90$enpres:a90$proximity1 : > numerical expression has 82 elements: only the first used3: In > analyze.model(term, mod, xlevels, default.levels) : > 0 does not appear in the modelError in > plot(effect(a90$enpres:a90$proximity1, mreg01)) : > error in evaluating the argument 'x' in selecting a method for function > 'plot' > > > > > Thanks in advance. > Tomas ______________________________________________ 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.
