Hi John. Thanks much for your help. It is great to know this. Hanna 2017-03-16 8:02 GMT-04:00 Fox, John <j...@mcmaster.ca>:
> Dear Hanna, > > You can test the slope in each non-reference group as a linear hypothesis. > You didn’t make the data available for your example, so here’s an example > using the linearHypothesis() function in the car package with the Moore > data set in the same package: > > - - - snip - - - > > > library(car) > > mod <- lm(conformity ~ fscore*partner.status, data=Moore) > > summary(mod) > > Call: > lm(formula = conformity ~ fscore * partner.status, data = Moore) > > Residuals: > Min 1Q Median 3Q Max > -7.5296 -2.5984 -0.4473 2.0994 12.4704 > > Coefficients: > Estimate Std. Error t value Pr(>|t|) > (Intercept) 20.79348 3.26273 6.373 1.27e-07 *** > fscore -0.15110 0.07171 -2.107 0.04127 * > partner.statuslow -15.53408 4.40045 -3.530 0.00104 ** > fscore:partner.statuslow 0.26110 0.09700 2.692 0.01024 * > --- > Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1 > > Residual standard error: 4.562 on 41 degrees of freedom > Multiple R-squared: 0.2942, Adjusted R-squared: 0.2426 > F-statistic: 5.698 on 3 and 41 DF, p-value: 0.002347 > > > linearHypothesis(mod, "fscore + fscore:partner.statuslow") > Linear hypothesis test > > Hypothesis: > fscore + fscore:partner.statuslow = 0 > > Model 1: restricted model > Model 2: conformity ~ fscore * partner.status > > Res.Df RSS Df Sum of Sq F Pr(>F) > 1 42 912.45 > 2 41 853.42 1 59.037 2.8363 0.09976 . > --- > Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1 > > - - - snip - - - > > In this case, there are just two levels for partner.status, but for a > multi-level factor you can simply perform more than one test. > > > I hope this helps, > > John > > ------------------------------------- > John Fox, Professor > McMaster University > Hamilton, Ontario, Canada > Web: http://socserv.mcmaster.ca/jfox/ > > > > > On 2017-03-15, 9:43 PM, "R-help on behalf of li li" > <r-help-boun...@r-project.org on behalf of hannah....@gmail.com> wrote: > > >Hi all, > > Consider the data set where there are a continuous response variable, a > >continuous predictor "weeks" and a categorical variable "region" with five > >levels "a", "b", "c", > >"d", "e". > > I fit the ANCOVA model as follows. Here the reference level is region > >"a" > >and there are 4 dummy variables. The interaction terms (in red below) > >represent the slope > >difference between each region and the baseline region "a" and the > >corresponding p-value is for testing whether this slope difference is > >zero. > >Is there a way to directly test whether the slope corresponding to each > >individual factor level is 0 or not, instead of testing the slope > >difference from the baseline level? > > Thanks very much. > > Hanna > > > > > > > > > > > > > >> mod <- lm(response ~ weeks*region,data)> summary(mod) > >Call: > >lm(formula = response ~ weeks * region, data = data) > > > >Residuals: > > Min 1Q Median 3Q Max > >-0.19228 -0.07433 -0.01283 0.04439 0.24544 > > > >Coefficients: > > Estimate Std. Error t value Pr(>|t|) > >(Intercept) 1.2105556 0.0954567 12.682 1.2e-14 *** > >weeks -0.0213333 0.0147293 -1.448 0.156 > >regionb -0.0257778 0.1349962 -0.191 0.850 > >regionc -0.0344444 0.1349962 -0.255 0.800 > >regiond -0.0754444 0.1349962 -0.559 0.580 > >regione -0.1482222 0.1349962 -1.098 0.280 weeks:regionb > >-0.0007222 0.0208304 -0.035 0.973 > >weeks:regionc -0.0017778 0.0208304 -0.085 0.932 > >weeks:regiond 0.0030000 0.0208304 0.144 0.886 > >weeks:regione 0.0301667 0.0208304 1.448 0.156 --- > >Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1 > > > >Residual standard error: 0.1082 on 35 degrees of freedom > >Multiple R-squared: 0.2678, Adjusted R-squared: 0.07946 > >F-statistic: 1.422 on 9 and 35 DF, p-value: 0.2165 > > > > [[alternative HTML version deleted]] > > > >______________________________________________ > >R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see > >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. > > [[alternative HTML version deleted]] ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see 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.
