# Too many categories

Contents

By using the same fun analysis as in my previous article, I would like too highlight a problem that often occurs, dealing with too much categories (or factors as they are called in R) avoid to see clearly the big pictures.

## The problem

To stick to my previous example, I want this time to check the Body Mass Index (BMI) by species. But in the starwars dataset there is a bunch of species, and it’s not easy to visualize the result – I have removed Jabba the Hutt since he crushes all the other categories that cannot be viewed at all, see my previous article for the full story.

starwars %>%
mutate(height = set_units(height, cm)) %>%
mutate(mass = set_units(mass, kg)) %>%
mutate(bmi = mass / set_units(height, m) ^ 2) %>%
select(name, height, mass, bmi, species) %>%
drop_na() %>%
filter(mass != max(mass)) %>% # Here is Jabba!
ggplot() +
geom_boxplot(aes(bmi, species, colour = species, fill = after_scale(alpha(colour, 0.5)))) +
theme_minimal()


## The solution

The solution is to lump together the less representative categories (in this case the species) in a big category often called Other. R , in the package forcats, provides a very convenient functions to perform this operation in different flavors fct_lump. I will use the fct_lump_n that

lumps all levels except for the n most frequent (or least frequent if n < 0)

So this means if I use mutate(species = forcats::fct_lump_n(species, n = 5)) I will end with the five most frequent species (species having the most individuals) plus the other species lumped together in the Other category.

Let’s check the result.

starwars %>%
mutate(height = set_units(height, cm)) %>%
mutate(mass = set_units(mass, kg)) %>%
mutate(bmi = mass / set_units(height, m) ^ 2) %>%
select(name, height, mass, bmi, species) %>%
drop_na() %>%
filter(mass != max(mass)) %>% # Here is Jabba!
# Here is the trick lumps all levels to "other" except for the n most frequent
mutate(species = forcats::fct_lump_n(species, n = 5)) %>%
ggplot() +
geom_boxplot(aes(bmi, species, colour = species, fill = after_scale(alpha(colour, 0.5)))) +
theme_minimal()


So it makes sense

• the human specie fall in the standard BMI range for humans,
• droids are heavier, it seems logical since they are made of metal as I far as I know.