[R-sig-ME] Changes of estimate sign when interaction effects are highly significant
raptorbio at hotmail.com
Tue Mar 5 19:06:58 CET 2013
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.
> 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?
> R-sig-mixed-models at r-project.org mailing list
More information about the R-sig-mixed-models