Dear Anne, Is age a continuous or a categorical variable? If it is a continuous variable, then I would add it to the fixed effects.
You have two options for the random effect: (Fertilier|Family/Species) or (1|Family/Species) + (0 + Fertilizer|Family/Species). Both yield a random intercept and a random slope along Fertilizer. But the main difference is the correlation between the random effects. The first model allows for correlation between the random effect and the random slope. Whereas the second model implies that they are independent. This is what I would do to test for the interaction. #If age is categorical M0 <- lmer(RGR ~ Fertilizer * Status + (1|Age) + (1|Family/Species) + (0 + Fertilizer|Family/Species), data=rgr, REML = TRUE) M1 <- lmer(RGR ~ Fertilizer * Status + (1|Age) + (1|Family/Species), data=rgr, REML = TRUE) anova(M0, M1) #If age is continuous M0 <- lmer(RGR ~ Fertilizer * Status + Age + (1|Family/Species) + (0 + Fertilizer|Family/Species), data=rgr, REML = TRUE) M1 <- lmer(RGR ~ Fertilizer * Status + Age + (1|Family/Species), data=rgr, REML = TRUE) anova(M0, M1) HTH, Thierry PS. The R-Sig-mixed models mailing list is better suited for questions on lme4. ------------------------------------------------------------------------ ---- ir. Thierry Onkelinx Instituut voor natuur- en bosonderzoek / Research Institute for Nature and Forest Cel biometrie, methodologie en kwaliteitszorg / Section biometrics, methodology and 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 Anne Kempel Verzonden: maandag 23 maart 2009 9:54 Aan: r-help@r-project.org Onderwerp: [R] Syntax in lmer... Sorry for bothering you - but I have a little problem. I`m completely new in R and trying to use lmer. That works, but I m not sure if my code is right, so I would just like to get an opinion from the experts. I have a Growthrate of plants (RGR Fixed Factors: Fertilizer, Status (alien or native plants) and their interaction Random Factors: Age is a covariable; than I have plantfamily and Species nested within Family. Now I want also to test for the interaction of Fertilizer with plantfamily and species. Did I do this the right way, and is the 0+ necessary? model<-lmer(RGR~Fertilizer+Status+Fertilizer:Status+(1|Age)+(1|Family/Sp ecies)+(0+Fertilizer|Family/Species),data=rgr) I`m thankful for any comments! anne -- Anne Kempel Institute of Plant Sciences University of Bern Altenbergrain 21 CH-3103 Bern kem...@ips.unibe.ch ______________________________________________ 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. 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.
