# Combining successive and treatment contrasts in lmer

I am running a lmer mixed effects model with three fixed effects parameters, each having multiple levels:

predic1: HH LR RR LD (4 levels) predic3: L1 L2 L3 L4 (4 levels) predic2: A B C D E F (5 levels)

I know that R, by default, uses the treatment coding for contrasts, where each level is compared to a reference level. I would instead like to have access to all the possible pairwise comparisons between different levels of each predictor. For example, in the case of predic1, I would like to see the following comparisons in the results of the model:

(HH-LR) 1-2 (HH-RR) 1-3 (HH-LD) 1-4 (LR-RR) 2-3 (LR-LD) 2-4 (RR-LD) 3-4

More like a combination of successive and treatment contrast codings.

Is there any built-in function in R for this type of comparison? If not what is the correct contrast matrix for these comparisons?

library(multcomp)

• You can set the degrees of freedom for the $t$ distribution (e.g., what you get from the lmerTest) in glht() using the df argument. – Dimitris Rizopoulos Oct 6 '18 at 18:00