Factors: Categorical Data in R

Interactive beginner R tutorial on factors: storing categories with factor(), levels(), setting level order, relevel() and why factors matter for models and plots. Run real R in your browser.

Groups, categories, treatment arms β€” R stores them as factors, and using them well makes your models and plots behave. Everything below is a live R console β€” edit the code and press Run Code.

Watch: factors in R

What a factor is, how to set its level order, and why it drives grouping in plots and models.

Download the R script  Β·  β–Ά Practice in the playground β€” or edit and run each snippet below.

A column that holds categories

The Species column of iris is a factor. levels() lists its categories; table() counts them:

Set a meaningful order

Build your own factor and fix the level order β€” this controls how categories are coded and displayed:

Factors drive grouping

Pass a factor on the right of a ~ formula and R splits the outcome by group automatically:

Choose the reference level

relevel() picks the baseline your models compare everything against:

Your turn

Next: Missing values (NA) β†’ β€” coping with the gaps in real data.