Understanding/plotting fixed effects estimates &standard errors
hi dr Duffy, many thanks for the attention and the reply. I followed the instructions under "Predictions and/or confidence (or prediction) intervals on predictions" of the wiki FAQ; however, unless I missed something, the results were nearly identical (and therefore confidence intervals wide & overlapping) to what I had by removing the intercept from the model:
m1=lmer(cbind(success,fail) ~ factor + (1|spp/variable), bb, family='binomial') summary(m1)
...
Fixed effects:
Estimate Std. Error z value Pr(>|z|)
(Intercept) 1.43834 0.52289 2.751 0.00595 **
factor1 0.21081 0.04269 4.938 7.88e-07 ***
newdat=expand.grid(success=0,fail=0,factor=c('0',"1"))
mm=model.matrix(terms(m1),newdat)
estims=mm %*% fixef(m1)
estims
[,1] 1 1.438342 2 1.649149
pvar1=diag(mm %*% tcrossprod(vcov(m1),mm)) sqrt(pvar1)
[1] 0.5228946 0.5229200 #compare to
m3=lmer(cbind(success,fail) ~ factor -1 + (1|spp/variable), bb, family='binomial') summary(m3)
...
Fixed effects:
Estimate Std. Error z value Pr(>|z|)
factor0 1.4383 0.5229 2.751 0.00595 **
factor1 1.6491 0.5229 3.154 0.00161 **
many thanks,
Abra?os,
Rafael Maia
---
webpage: http://gozips.uakron.edu/~rm72
"A little learning is a dangerous thing; drink deep, or taste not the Pierian spring." (A. Pope)
Graduate Student - Integrated Bioscience
University of Akron
http://gozips.uakron.edu/~shawkey/
On Jun 21, 2012, at 10:48 PM, David Duffy wrote:
This is nothing to with mixed modelling per se, but how your contrasts for the fixed effects are set up. If this was fixed effects only, you could fit -1 + factor (no intercept) to give estimates for each level of factor with standard errors that you could correctly interpret as you are trying to. The FAQ (http://glmm.wikidot.com/faq) shows plotting of confidence intervals for predictions.