Visualising variance from random effects in a mixed model by group

Question

I have run a linear mixed model in R using lmer. I am attempting to visualise the random effect structure. To produce a graph I have used print(dotplot(ranef(RT.model.4, condVar=T))[['part_no']]) where part_no is the random effect from the mixed model. It creates something like this:

This is great. However I want to be able to visually tell the difference between my two groups of participants (the random effect being discussed) in the graph. I have group A and group B. In my dataset I have a column for participant type and for each row it gives a value of A or B.

I would like to either colour code the graph to show participants from groups A and B. Or perhaps better would be to create two separate panels, one for each group.

Any suggestions on how to do this would be very much appreciated.

Answer 1

This is a way using ggplot rather than lattice (just because I am more familiar with it) using code from the examples in ?dotplot.ranef.mer . You need to match your treatment group in the data to the random effects grouping variables returned by ranef . I don't see how this can be done automatically within dotplot.ranef.mer .

Create a small example with a treatment group; each subject is assigned to one treatment group.

library(lme4)
library(ggplot2)

sleepstudy$trt = as.integer(sleepstudy$Subject %in% 308:340)

m = lmer(Reaction ~  trt + (1|Subject), sleepstudy)

Convert the random effects to a dataframe and match in the treatment groups

dd = as.data.frame(ranef(m, condVar=TRUE), "Subject")
dd$trt = with(sleepstudy, trt[match(dd$grp, Subject)])

You can then plot how you want, say using facet_ 's or assigning a colour to each group, or...

ggplot(dd, aes(y=grp,x=condval, colour=factor(trt))) +
        geom_point() + facet_wrap(~term,scales="free_x") +
        geom_errorbarh(aes(xmin=condval -2*condsd,
                           xmax=condval +2*condsd), height=0)

ggplot(dd, aes(y=grp,x=condval)) +
        geom_point() +
        geom_errorbarh(aes(xmin=condval -2*condsd,
                           xmax=condval +2*condsd), height=0)+
        facet_wrap(~trt)

Answer 2

You should be able to use the groups= option in dotplot() . Assuming your data is in a dataframe called df with the group variable being in group , you could use

print(dotplot(ranef(RT.model.4, condVar=T), groups=df$group)[['part_no']])