Factors

Lecture 14

Dr. Eric Friedlander

College of Idaho
CSCI 2025 - Winter 2026

Introduction

Categorical Data and Factors

  • Factors are R’s data structure for categorical variables.
  • The forcats package (part of tidyverse) provides a suite of tools for working with factors.
  • Useful for reordering and relabeling for better visualizations.

Creating Factors

factor() and fct()

  • Create factors from a character vector.
  • levels() shows the categories of the factor.
  • The levels have an order (not to be confused with “ordered factors” which are different)
x <- c("Male", "Female", "Female", "Male")
f <- factor(x)
f
[1] Male   Female Female Male  
Levels: Female Male
levels(f)
[1] "Female" "Male"

Data: Idaho Naturalization

  • We’re going to use Naturalization data from the Idaho State Archives again
  • This dataset was assembled by Dr. Rachel Miller
  • Link to Google Sheet
  • Let’s load the data and convert the country of origin to a factor

Modifying Factor Order

fct_reorder()

  • Reorders a factor by another variable, useful for plots.
  • For example, ordering bars in a bar chart.
  • Let’s create a bar chart of country of origin ordered by the average year of arrival.

fct_infreq()

  • Reorders a factor by the frequency of its values.
  • Let’s create a bar chart of country of origina by the frequency

fct_relevel()

  • Manually move levels to the front.
  • Let’s move “Spain” to the front.

Modifying Factor Levels

  • fct_recode(): Changes the names of factor levels.
  • fct_collapse(): Combines several levels into one.
  • fct_lump_*(): lump together the least/most frequent levels into an “Other” category.

Practice

  • Let’s make the country of origin factor more manageable by lumping together the least frequent countries into “Other” and recoding some country names for clarity.

Ordered Factors

  • A factor that enforces a strict ordering of levels
  • Useful for ordinal data (e.g., ratings)
  • This class: only really matters if you tie it to a color aesthetic
    • Will select a color scheme that implies an ordering

Wrap Up

Do Next

  1. Read Chapter 16: Factors from r4ds.
  2. Open the Recitation Gem and say “Provide me practice problems for Chapter 16” or work through some of the exercises in the text.
  3. Move on to lecture 15.