lognormal_with_mean_parameter

# based on the great vignette on specifying custom response distributions in brms:
# https://cran.r-project.org/web/packages/brms/vignettes/brms_customfamilies.html

library(languageR)
library(brms)

danish$RT <- exp(danish$LogRT) 
danish$cSex <- ifelse(danish$Sex == "M", -1, 1) * 0.5 

lognormal2 <- custom_family(
  "lognormal2", dpars = c("mu", "sigma"),
  links = c("identity", "log"), lb = c(0, 0),
  type = "real"
)

stan_funs <- "
real lognormal2_lpdf(real y, real mu, real sigma) {
  real real_mu = log(mu) - sigma^2/2;
  return lognormal_lpdf(y | real_mu, sigma);
}
real lognormal2_rng(real mu, real sigma) {
  real real_mu = log(mu) - sigma^2/2;
  return lognormal_rng(real_mu, sigma);
}
"

stanvars <- stanvar(scode = stan_funs, block = "functions")

m <- brm(RT ~ cSex, data = danish, family = lognormal2, stanvars = stanvars, chains = 1)
summary(m)