[R-sig-ME] Changes of estimate sign when interaction effects are highly significant

[Adam Smith]

Teresa,

Correct, main effects cannot be interpreted in the presence of an interaction, unless the main effect input variables are centered...

See, for example:

Engqvist, L. (2005) The mistreatment of covariate interaction terms in linear model analyses of behavioural and evolutionary ecology studies. Animal Behaviour, 70, 967–971. 

(free pdf here: http://pub.uni-bielefeld.de/download/2395676/2496935)

Schielzeth H (2010) Simple means to improve the interpretability of regression coefficients. Methods in Ecology and Evolution, 1, 103–113. 

(http://onlinelibrary.wiley.com/doi/10.1111/j.2041-210X.2010.00012.x/full)


Cheers,

Adam Smith


> Date: Tue, 5 Mar 2013 18:56:07 +0100
> From: tmoranlopez at mncn.csic.es
> To: r-sig-mixed-models at r-project.org
> Subject: [R-sig-ME] Changes of estimate sign when interaction effects are highly significant
> 
> Hi everyone,
> I am trying to evaluate the effects of WUE and drought on trees 
> growth. I am running a mixed model in which my dependent variable is 
> growth and my fixed effects are WUE, drought and their interaction. My 
> random variables treeID nested in year. When the interaction among the 
> two variables is not included, the effects of WUE on growth is 
> negative and the effects of drought negative which is very intuitive. 
> However, the best model includes the interaction (which is highly 
> significant), and then the sign of the main effects change, now both 
> drought and WUE have a positive effect on growth! And the sign of the 
> interaction is negative...
> Does anyone know what could be happening? Does this mean that when we 
> have a very significant interaction between two fixed effects they 
> cannot be interpreted separately?
> Thanks
> 
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