Re: [R] mixed-effects model with two fixed effects: interaction

From: Bilonick, Richard A <bilonickra_at_upmc.edu>
Date: Tue, 29 Jun 2010 09:41:33 -0400

On Tue, 2010-06-29 at 09:09 +0000, Ilona Leyer wrote:
> Dear all,
> In a greenhouse experiment we tested performance of 4 different species (B,H,P,R) under 3 different water levels in 10 replications. As response variable e.g. the number of emerging sprouts were measured on three dates. A simple Anova considering every measurement date separately shows a higly significant effect of species and moisture (and partly the interaction of both). The mixed-effects model with species and moisture shows a highly significant effect of species and moisture as well. However, when I included the interaction the t-values of the species dropped strongly and the SE increase and the results for the species are not significant anymore. For me this does not seem plausible. Has anybody an idea, how this can be interpreted and if I have done a mistake in calculating the data?
>
> Thanks in advance for any help!
> Ilona
>
>
> model1<-lme(sprouts~species+moisture,random=~time|ID)
> model2<-lme(sprouts~species*moisture,random=~time|ID)
>
>
> Fixed effects: sprouts ~ species + moisture
> Value Std.Error DF t-value p-value
> (Intercept) 7.971267 1.330500 240 5.991180 0.0000
> speciesH -6.459344 1.536329 114 -4.204400 0.0001
> speciesP -10.063604 1.536329 114 -6.550421 0.0000
> speciesR -5.051894 1.536329 114 -3.288288 0.0013
> moisturemoist 2.228835 1.330500 114 1.675185 0.0966
> moisturewaterlogged 17.111149 1.330500 114 12.860688 0.0000
>
>
> Fixed effects: sprouts ~ species * moisture
> Value Std.Error DF t-value p-value
> (Intercept) 4.831965 1.750970 240 2.759594 0.0062
> speciesH -4.464197 2.476245 108 -1.802809 0.0742
> speciesP -3.986787 2.476245 108 -1.610013 0.1103
> speciesR -0.809376 2.476245 108 -0.326856 0.7444
> moisturemoist 3.505506 2.476245 108 1.415654 0.1598
> moisturewaterlogged 24.766934 2.476245 108 10.001811 0.0000
> speciesH:moisturemoist -0.457291 3.501939 108 -0.130582 0.8963
> speciesP:moisturemoist -2.458125 3.501939 108 -0.701932 0.4842
> speciesR:moisturemoist -2.555356 3.501939 108 -0.729697 0.4672
> speciesH:moisturewaterlogged -5.597498 3.501939 108 -1.598400 0.1129
> speciesP:moisturewaterlogged -15.538272 3.501939 108 -4.437048 0.0000
> speciesR:moisturewaterlogged -10.206874 3.501939 108 -2.914635 0.0043
>
>
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When there is an interaction effect, the main effects are difficult to interpret. Your model is not a simple additive one when there is an interaction. You can't predict the level of one factor without knowing the level of the other factor. Given there is an interaction between these factors, you could reparameterize it as a one-way analysis (i.e., just create 12 separate treatment groups). When there is an interaction, you can't get a simple interpretation with just two factors.



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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. Received on Tue 29 Jun 2010 - 13:45:42 GMT

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