3  Summarising and displaying categorical data

Two common statistics are the number of categories and the mode, which is the most common value.

Example: Pixar films

Release, runtime, and critical reception data for Pixar’s feature films (see here for a brief description of the dataset).

number film release_date run_time film_rating rotten_tomatoes metacritic cinema_score critics_choice
1 Toy Story 1995-11-22 81.000000 G 100.000000 95.000000 A nan
2 A Bug's Life 1998-11-25 95.000000 G 92.000000 77.000000 A nan
3 Toy Story 2 1999-11-24 92.000000 G 100.000000 88.000000 A+ 100.000000
4 Monsters, Inc. 2001-11-02 92.000000 G 96.000000 79.000000 A+ 92.000000
5 Finding Nemo 2003-05-30 100.000000 G 99.000000 90.000000 A+ 97.000000
6 The Incredibles 2004-11-05 115.000000 PG 97.000000 90.000000 A+ 88.000000
7 Cars 2006-06-09 117.000000 G 74.000000 73.000000 A 89.000000
8 Ratatouille 2007-06-29 111.000000 G 96.000000 96.000000 A 91.000000
9 WALL-E 2008-06-27 98.000000 G 95.000000 95.000000 A 90.000000
10 Up 2009-05-29 96.000000 PG 98.000000 88.000000 A+ 95.000000
11 Toy Story 3 2010-06-18 103.000000 G 98.000000 92.000000 A 97.000000
12 Cars 2 2011-06-24 106.000000 G 40.000000 57.000000 A- 67.000000
13 Brave 2012-06-22 93.000000 PG 78.000000 69.000000 A 81.000000
14 Monsters University 2013-06-21 104.000000 G 80.000000 65.000000 A 79.000000
15 Inside Out 2015-06-19 95.000000 PG 98.000000 94.000000 A 93.000000
16 The Good Dinosaur 2015-11-25 93.000000 PG 76.000000 66.000000 A 75.000000
17 Finding Dory 2016-06-17 97.000000 PG 94.000000 77.000000 A 89.000000
18 Cars 3 2017-06-16 102.000000 G 69.000000 59.000000 A 66.000000
19 Coco 2017-11-22 105.000000 PG 97.000000 81.000000 A+ 89.000000
20 Incredibles 2 2018-06-15 118.000000 PG 93.000000 80.000000 A+ 86.000000
21 Toy Story 4 2019-06-21 100.000000 G 97.000000 84.000000 A 94.000000
22 Onward 2020-03-06 102.000000 PG 88.000000 61.000000 A- 79.000000
23 Soul 2020-12-25 100.000000 PG 96.000000 83.000000 nan 93.000000
24 Luca 2021-06-18 151.000000 nan nan nan nan nan
25 Turning Red 2022-03-11 nan nan nan nan nan nan
26 Lightyear 2022-06-17 nan nan nan nan nan nan
27 Elemental 2023-06-16 155.000000 Not Rated nan nan nan nan

Two of the categorical variables in the Pixar dataset are film_rating and cinema_score. For each, we can report the number of categories and the mode:

variable number of categories mode
film_rating 3 G
cinema_score 3 A

film_rating has 3 categories, and the most common one is G. cinema_score also has 3 categories, and the most common one is A.

A more complete picture is provided by the frequency table, which lists the categories of a categorical variable and the count of observations for each category.

For example, the frequency table for cinema_score:

cinema_score count
A 13
A+ 7
A- 2

A relative frequency table displays percentages (or proportions) rather than the counts in each category. Both types of tables show the distribution of a categorical variable because they list the possible categories and indicate how often each occurs.

For example, the relative frequency table for cinema_score:

cinema_score percentage
A 59.100000
A+ 31.800000
A- 9.100000

A visual representation of the frequency table: bar charts.

For example, here are the counts and the percentages for cinema_score displayed as bar charts:

Bar charts are easier than tables to compare frequencies: easy to see the mode and the 2nd most common category, and that there are twice as many observations in the most common category as in the second most common.

There are a few other representations of the distribution, but none are as easy to understand as bar charts.

Pie charts. The same cinema_score distribution, as a pie chart:

Pie charts are harder to read than bar charts.

Warning
  • Don’t try to interpret the shape of a bar chart. Category order is arbitrary in a bar chart.
  • A plot should always have labels of the x-axis and y-axis (with units).