Hi, I?ve got this model> model<-glm(prevalence~agesex+agesex:month,binomial)and the output of anova is like that> anova(model,test="Chisq")Df Deviance Resid. Df Resid. Dev P(>|Chi|) NULL 524 206.97 agesex 2 9.9165 522 197.05 0.007025 ** agesex:month 9 18.0899 513 178.96 0.034145 * I don?t know how to interpret the interaction "agesex:month", my mind doubt between 2 options: a) For a giving group of "agesex" there are differences between months b)There are differences between groups of "agesex" in some months but not in others. Which option is correct? Thanks very much ----- Mario Garrido Escudero PhD student Dpto. de Biolog?a Animal, Ecolog?a, Parasitolog?a, Edafolog?a y Qca. Agr?cola Universidad de Salamanca -- View this message in context: http://r.789695.n4.nabble.com/interpreting-interactions-in-a-model-tp3749430p3749430.html Sent from the R help mailing list archive at Nabble.com.
Hi: On Wed, Aug 17, 2011 at 1:56 AM, gaiarrido <gaiarrido at usal.es> wrote:> Hi, > I?ve got this model >> model<-glm(prevalence~agesex+agesex:month,binomial) > > and the output of anova is like that > >> anova(model,test="Chisq") > ? ? ? ? ? ? ? ? ? ? ? ? ?Df Deviance Resid. Df Resid. Dev P(>|Chi|) > NULL ? ? ? ? ? ? ? ? ? ? ? ? ? ?524 ? ? 206.97 > agesex ? ? ? ? ? ? ? ? 2 ? 9.9165 ? ? ? 522 ? ? 197.05 ?0.007025 ** > agesex:month ? ? ? ?9 ?18.0899 ? ? ? 513 ? ? 178.96 ?0.034145 *Where is the month main effect? Even if it's 'not significant', it belongs in the model (principle of marginality).> > I don?t know how to interpret the interaction "agesex:month", my mind doubt > between 2 options: > a) For a giving group of "agesex" there are differences between months > b)There are differences between groups of "agesex" in some months but not in > others. > > Which option is correct?In the absence of a reproducible example, no one can say, but it's within the realm of possibility that either or both could be correct. The significance test for interaction indicates *whether* an interaction effect exists - it doesn't tell you *what type* of interaction exists. Conditional on the inference that an interaction effect is present, the next step of the analysis is to investigate the nature of the interaction. This could involve planned contrasts, multiple comparisons, graphics, etc. The car, effects, multcomp and HH packages can be useful for these types of investigations. HTH, Dennis> Thanks very much > > ----- > Mario Garrido Escudero > PhD student > Dpto. de Biolog?a Animal, Ecolog?a, Parasitolog?a, Edafolog?a y Qca. Agr?cola > Universidad de Salamanca > -- > View this message in context: http://r.789695.n4.nabble.com/interpreting-interactions-in-a-model-tp3749430p3749430.html > Sent from the R help mailing list archive at Nabble.com. > > ______________________________________________ > R-help at 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. >
The Predict.Plot function in the TeachingDemos package can help you visualize interactions. It will work best if Month is treated as a continuous variable. -- Gregory (Greg) L. Snow Ph.D. Statistical Data Center Intermountain Healthcare greg.snow at imail.org 801.408.8111> -----Original Message----- > From: r-help-bounces at r-project.org [mailto:r-help-bounces at r- > project.org] On Behalf Of gaiarrido > Sent: Wednesday, August 17, 2011 2:57 AM > To: r-help at r-project.org > Subject: [R] interpreting interactions in a model > > Hi, > I?ve got this model > > model<-glm(prevalence~agesex+agesex:month,binomial) > > and the output of anova is like that > > > anova(model,test="Chisq") > Df Deviance Resid. Df Resid. Dev P(>|Chi|) > NULL 524 206.97 > agesex 2 9.9165 522 197.05 0.007025 ** > agesex:month 9 18.0899 513 178.96 0.034145 * > > I don?t know how to interpret the interaction "agesex:month", my mind > doubt > between 2 options: > a) For a giving group of "agesex" there are differences between months > b)There are differences between groups of "agesex" in some months but > not in > others. > > Which option is correct? > Thanks very much > > ----- > Mario Garrido Escudero > PhD student > Dpto. de Biolog?a Animal, Ecolog?a, Parasitolog?a, Edafolog?a y Qca. > Agr?cola > Universidad de Salamanca > -- > View this message in context: > http://r.789695.n4.nabble.com/interpreting-interactions-in-a-model- > tp3749430p3749430.html > Sent from the R help mailing list archive at Nabble.com. > > ______________________________________________ > R-help at 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.