The goal of lmereffect is to calculate Cohen's d for a dichotomous variable in a model fitted with lme4::lmer()
You can install lmereffect from github with:
# install.packages("devtools")
devtools::install_github("jrosen48/lme4_effect")
This is a basic example which shows you how to solve a common problem:
#' library(lme4)
#' sleepstudy$dichotomous_var <- rbinom(nrow(sleepstudy), 1, .5)
#' fm1 <- lmer(Reaction ~ Days + dichotomous_var + (1 | Subject), sleepstudy)
#' lmer_effect(df = sleepstudy, Reaction, fm1, dichotomous_var)