7  Box plots

This graph is largely complete and just needs final proof reading.

This graph requires:

✅ a numeric (continuous) variable

7.1 Description

Box plots use five values to summarize data: minimum, first quartile, median, third quartile, and maximum. The plot looks like a box with a line in the middle. Whiskers go from the box to the minimum and maximum values. Unusual points are shown as dots outside the whiskers. Box plots help identify unusual points and compare different groups of data.

7.2 Set up

PACKAGES:

Install packages.

install.packages("palmerpenguins")
library(palmerpenguins) 
library(ggplot2)

DATA:

Artwork by Allison Horst

We’ll be using the penguins data from palmerpenguins.

penguins <- palmerpenguins::penguins 
glimpse(penguins)
#> Rows: 344
#> Columns: 8
#> $ species           <fct> Adelie, Adelie, Adelie…
#> $ island            <fct> Torgersen, Torgersen, …
#> $ bill_length_mm    <dbl> 39.1, 39.5, 40.3, NA, …
#> $ bill_depth_mm     <dbl> 18.7, 17.4, 18.0, NA, …
#> $ flipper_length_mm <int> 181, 186, 195, NA, 193…
#> $ body_mass_g       <int> 3750, 3800, 3250, NA, …
#> $ sex               <fct> male, female, female, …
#> $ year              <int> 2007, 2007, 2007, 2007…

7.3 Grammar

CODE:

Create labels with labs()

Initialize the graph with ggplot() and provide data

• Assign a blank character string ("") to the x axis in labs()

Map flipper_length_mm to the y axis and an empty string ("") to the x axis

Add the geom_boxplot() layer

labs_boxplot <- labs(
  title = "Adult foraging penguins",
  subtitle = "Distribution of flipper length",
  x = "",
  y = "Flipper length (millimeters)")
ggp2_boxplot <- ggplot(data = penguins,
           aes(x = "", 
               y = flipper_length_mm)) +
        geom_boxplot() 
ggp2_boxplot + 
  labs_boxplot

GRAPH:

7.3.1 More Info

Below is more information on interpreting Box plots.

7.3.2 movies data

We’ll use the ggplot2movies::movies data to create a box plot for movie length

install.packages("ggplot2movies")
library(ggplot2movies) 
library(ggplot2)

Filter ggplot2movies::movies to only include films after the made after 2000, and remove missing values from mpaa and budget

movies_box <- ggplot2movies::movies |> 
                dplyr::filter(year > 2000 & 
                                mpaa != "" & 
                                    !is.na(budget))
head(movies_box)

Below we create a box plot of the length variable using the methods described above:

labs_boxplot <- labs(
  title = "IMDB Movie information and user ratings",
  y = "Movie length (min)", x = "")
ggp2_boxplot <- ggplot(data = movies_box, 
           aes(x = " ", 
               y = length)) +
        geom_boxplot() 

ggp2_boxplot + 
  labs_boxplot

7.3.3 Interpret

The table below shows the 25th percentile, the median, the 75th percentile, the IQR, and a histogram of the length variable from the movies_box dataset.

25th	Median	75th	IQR	Histogram
92	100	113	21	▁▇▅▁▁

The figure below displays how each element in the box plot represents each of the statistics using lines and points.

In ggplot2, values that fall more than 1.5 times the IQR are displayed as individual points (aka outliers). The lines extending from the bottom and top of the main box represent the last non-outlier value in the distribution.

7.3.4 Compare

Compare the geom_point(), geom_freqpoly(), geom_histogram(), and geom_density() graphs of length from movie_box below to the geom_boxplot():