Expand check_heterogeneity_bias()'s output

[strengejacke]

Fixes #810

[mattansb]

Okay, I like this a lot:

library(performance)

mlmRev::egsingle |> 
  check_group_variation(select = c("lowinc", "female", "math"),
                        by = c("schoolid", "childid"))
#> Check group variation
#> 
#> group    | variable |    type
#> -----------------------------
#> schoolid |   lowinc | between
#> schoolid |   female |  within
#> schoolid |     math |    both
#> childid  |   lowinc | between
#> childid  |   female | between
#> childid  |     math |    both


mlmRev::egsingle |> 
  check_group_variation(select = c("lowinc", "female", "math"),
                        by = c("schoolid", "childid"), 
                        
                        include_by = TRUE)
#> Check group variation
#> 
#> group    | variable |    type
#> -----------------------------
#> schoolid |  childid |  nested
#> schoolid |   lowinc | between
#> schoolid |   female |  within
#> schoolid |     math |    both
#> childid  | schoolid | between
#> childid  |   lowinc | between
#> childid  |   female | between
#> childid  |     math |    both



dat <- data.frame(
  id = rep(letters, each = 2),
  
  constant = "a",
  
  between_num = rep(rnorm(26), each = 2),
  within_num = rep(rnorm(2), times = 26),
  both_num = rnorm(52),
  
  between_fac = rep(LETTERS, each = 2),
  within_fac = rep(LETTERS[1:2], times = 26),
  both_fac = sample(LETTERS[1:5], size = 52, replace = TRUE)
)


dat |> 
  check_group_variation(by = "id")
#> Check id variation
#> 
#> variable    |    type
#> ---------------------
#> constant    |        
#> between_num | between
#> within_num  |  within
#> both_num    |    both
#> between_fac | between
#> within_fac  |  within
#> both_fac    |    both

^{Created on 2025-05-07 with reprex v2.1.1}

I re-wrote a lot of the docs, to explain what is going on.

• @strengejacke How do I make the printing method split by group?
• Can this function replace (supersede) check_heterogeneity_bias()? If so, some of the docs from there can make their way into here.