add unit test

[tjmahr]

add a test that compares manual correlation calculation with results of draw_var_corr

library(rstanarm)
m4 <- stan_glmer(
  Reaction ~ Days + (Days | Subject),
  data = lme4::sleepstudy,
  prior_covariance = decov(1, 1, 1), 
  prior = normal(0, 1),
  prior_PD = TRUE,
  chains = 1)

me <- tristan::draw_var_corr(m4)
df <- as.data.frame(m4)

cors <- df$`Sigma[Subject:Days,(Intercept)]` /
  (sqrt(df$`Sigma[Subject:Days,Days]`) * sqrt(df$`Sigma[Subject:(Intercept),(Intercept)]`))

plot(density(cors, adjust = .2))
hist(cors)

cors2 <- me %>% filter(!is.na(var2)) %>% pull(sdcor)
plot(density(cors2, adjust = .2))
hist(cors2)