[R-meta] Testing of moderators in rma()
Dear all, I have a problem in finding the right test for the inclusion of moderators, or actually I'm not sure if I should include the intercept term or not. What troubles me, is that the removal of the intercept term, has a very big effect on the omnibus test of the moderators. The model: rma.mv() with an additional random effect (study), a variance-covariance matrix for the sampling variances and covariances (Lajeunesse correction). I want to test species as a moderator. When I include the intercept, the moderator effect is not significant (P=0.2779), and when I remove the intercept P<0.001. I started to remove the intercept to get the average effects for levels for each species and the z-test for each species. However, no I'm not sure anymore, what the different interpretation of moderator test for the two different models are. Thanks a lot! ### Model with intercept: > specmodel <- rma.mv(yi~species,V=varmat,random=~1|study/myo,data=metadat,method="REML") > summary(specmodel) Multivariate Meta-Analysis Model (k = 166; method: REML) ? logLik? Deviance?????? AIC?????? BIC????? AICc ?12.8545? -25.7089?? 22.2911?? 93.5666?? 32.3751 Variance Components: ??????????? estim??? sqrt? nlvls? fixed???? factor sigma^2.1? 0.0216? 0.1470???? 39???? no????? study sigma^2.2? 0.0300? 0.1732??? 166???? no? study/myo Test for Residual Heterogeneity: QE(df = 144) = 1386.5618, p-val < .0001 Test of Moderators (coefficient(s) 2:22): QM(df = 21) = 24.3187, p-val = 0.2779 ### Model without intercept: > specmodel <- rma.mv(yi~species-1,V=varmat,random=~1|study/myo,data=metadat,method="REML") > summary(specmodel) Multivariate Meta-Analysis Model (k = 166; method: REML) ? logLik? Deviance?????? AIC?????? BIC????? AICc ?12.8545? -25.7089?? 22.2911?? 93.5666?? 32.3751 Variance Components: ??????????? estim??? sqrt? nlvls? fixed???? factor sigma^2.1? 0.0216? 0.1470???? 39???? no????? study sigma^2.2? 0.0300? 0.1732??? 166???? no? study/myo Test for Residual Heterogeneity: QE(df = 144) = 1386.5618, p-val < .0001 Test of Moderators (coefficient(s) 1:22): QM(df = 22) = 61.9539, p-val < .0001
Samuel Knapp Lehrstuhl f?r Pflanzenern?hrung Technische Universit?t M?nchen (Chair of Plant Nutrition Technical University of Munich) Emil-Ramann-Strasse 2 D-85354 Freising Tel. +49 8161 71-3578 samuel.knapp at tum.de www.researchgate.net/profile/Samuel_Knapp