Exploratory Data Analysis, Part I

Lecture 2

John Zito

Duke University
STA 101 Fall 2026

2026-09-01

Last week

The shape of the course

Study these carefully!

Every main idea, intellectual theme, and computational skill makes an appearance:

• Lecture 1: “All of STA 101 in 75 minutes;”
• Lab 0: your coding “one-stop-shop.”

Study these carefully and return to them often. By December 4, it will all make sense!

Dispatches from the intro survey

Getting to know you

	No	Yes
AP Stats?	58.43%	41.57%
Prior coding?	52.33%	47.67%
Friends?	49.43%	50.57%

What’s the last movie you saw in a theater?

Today

What are we studying?

• Data analysis
  • The art of transforming messy, incomplete, imperfect data into knowledge;
  • Knowledge often takes the form of pictures and a concise set of numerical summaries;
• Statistical Inference
  • Quantifying our uncertainty about that knowledge.

Let’s focus on the first part

How do you compress a dataset down to pictures and numerical summaries?

• What kinds of pictures and summaries are available?
• How do you know what to use, and when?
• How do you get a computer to implement these things?

The type of a variable (numerical, categorical, etc) plays a big role in determining the menu of options. After that, good visualization and summarization is an art that requires good taste.

Our running example

Alison Bechdel

The Bechdel Test

Measures female representation in film:

(Dykes to Watch Out For - 1985)

Film passes if…

1. two female characters;
2. talk to each other;
3. about something besides a man.

Since I moved to Durham…

It’s been a pitiful two years of filmgoing:

Title	Year	JZ’s review	Bechdel
Conclave	2024	★★☆☆☆	❌
Wicked 1	2024	★★☆☆☆	✅
Nosferatu	2024	★★★★☆	❌
Naked Gun	2025	★★★☆☆	❌
Bugonia	2025	★★★☆☆	❌
Wicked 2	2025	★☆☆☆☆	✅
Marty Supreme	2026	★★★★☆	❌
Obsession	2026	★★★★☆	❌
The Odyssey	2026	★★☆☆☆	❌
Coyote vs. Acme	2026	★★★☆☆	❌

Go to your container

1. Visit the Duke Container Manager (and log in with your NetID);
2. (If you haven’t already, reserve the STA101 container on the right-hand-side)
3. Click STA101 under “My reservations” on the left-hand side;
4. Login, start, and give it a sec to launch in your browser.

Grab today’s files

• Double-check that you have the course project loaded in the upper right corner;
• Go to the Git tab in the upper right pane and click Pull.

Load today’s package(s)

It’s just the one today:

library(tidyverse)

Today’s data

bechdel <- read_csv("data/bechdel.csv")

bechdel

# A tibble: 1,615 × 7
   title                   year gross_2013 budget_2013    roi binary clean_test
   <chr>                  <dbl>      <dbl>       <dbl>  <dbl> <chr>  <chr>     
 1 21 & Over               2013   67878146    13000000  5.22  FAIL   notalk    
 2 Dredd 3D                2012   55078343    45658735  1.21  PASS   ok        
 3 12 Years a Slave        2013  211714070    20000000 10.6   FAIL   notalk    
 4 2 Guns                  2013  208105475    61000000  3.41  FAIL   notalk    
 5 42                      2013  190040426    40000000  4.75  FAIL   men       
 6 47 Ronin                2013  184166317   225000000  0.819 FAIL   men       
 7 A Good Day to Die Hard  2013  371598396    92000000  4.04  FAIL   notalk    
 8 About Time              2013  102648667    12000000  8.55  PASS   ok        
 9 Admission               2013   36014634    13000000  2.77  PASS   ok        
10 After Earth             2013  304895295   130000000  2.35  FAIL   notalk    
# ℹ 1,605 more rows

Aside: variable assignment <-

You just saw me do this:

bechdel <- read_csv("data/bechdel.csv")

• This is called variable assignment. We’re saving an object so we can use it later;
• Right-hand side: the thing you want to store (the data);
• Left-hand side: the name you want to give it so you can refer to it later;
• In the middle: the assignment operator <-.

Our variables

Every row is a film, and the columns include:

• title: name of movie
• year: release year of movie (between 1990 and 2013);
• gross_2013: how much did the movie earn at the box office (in 2013 $);
• budget_2013: how much did the movie cost to make (in 2013 $);
• roi: Return on investment, calculated as the ratio of the gross to budget;
  • If the movie broke even, roi is 1;
  • If the movie made money, roi is greater than 1;
  • If the movie lost money, roi is between 0 and 1;
• clean_test: Bechdel test result:
  • ok = passes test;
  • dubious = unclear;
  • men = women only talk about men
  • notalk = women don’t talk to each other;
  • nowomen = fewer than two women;
• binary: Bechdel Test PASS vs FAIL binary

Can we reproduce this claim?

From FiveThirtyEight

“We did a statistical analysis of films to test two claims: first, that films that pass the Bechdel test — featuring women in stronger roles — see a lower return on investment, and second, that they see lower gross profits. We found no evidence to support either claim.”

🚨 Warning: resist the causal temptation!

These data are purely observational. No experiment was done, and I don’t think “nature” delivered any conditions that mimic experimental variation. We will only be able to talk about the mere association between female representation and box office performance.

Exploring a single numerical variable

The films’ grosses

The box office revenue for the films is a numerical variable:

bechdel$gross_2013

 [1]  67878146  55078343 211714070 208105475 190040426 184166317 371598396
 [8] 102648667  36014634 304895295

• A human cannot stare at this list of numbers and learn anything. We need pictures and summaries. So…what pictures? what summaries? what are we even trying to learn?
• We want to know where these numbers are typically concentrated, how spread out are they, and what is their shape;
• A histogram can answer those questions.

Plotting a histogram

Start with a blank canvas:

ggplot(bechdel)

Plotting a histogram

What variables are we looking at?

ggplot(bechdel, 
       aes(x = gross_2013))

The aesthetic mapping is where we tell the computer what variables in the data frame we want to use and how we want to use them.

Plotting a histogram

What kind of plot for this numerical variable:

ggplot(bechdel, 
       aes(x = gross_2013)) + 
  geom_histogram()

Plotting a histogram

Add labels:

ggplot(bechdel, 
       aes(x = gross_2013)) + 
  geom_histogram() + 
  labs(
    x = "2013 USD",
    title = "Film grosses (1990 - 2013)"
  )

Y’know…so we know what we’re looking at!

Plotting a histogram

Increase font size and give it a minimalist look:

ggplot(bechdel, 
       aes(x = gross_2013)) + 
  geom_histogram() + 
  labs(
    x = "2013 USD",
    title = "Film grosses (1990 - 2013)"
  ) +
  theme_minimal(base_size = 20)

Dataviz with ggplot is like building a cake

• Every plot element is a new layer, and you stack ’em up;
• If the different commands are like the layers of sponge, then the plus signs in between are like the icing. Don’t forget the icing!

The grammar of graphics

This design philosophy is described in the book The Grammar of Graphics and implemented in the ggplot2 package.

How is a histogram drawn?

1. Start with the number line (horizontal axis);

How is a histogram drawn?

1. Start with the number line (horizontal axis);
2. Put your data values on the line;

How is a histogram drawn?

1. Start with the number line (horizontal axis);
2. Put your data values on the line;
3. Break the line into bins;

How is a histogram drawn?

1. Start with the number line (horizontal axis);
2. Put your data values on the line;
3. Break the line into bins;
4. Count how many data values fall in each bin;

How is a histogram drawn?

1. Start with the number line (horizontal axis);
2. Put your data values on the line;
3. Break the line into bins;
4. Count how many data values fall in each bin;
5. Put bars over the bins (height = count).

Choice: how do you draw the bins? How many? How wide?

Back to Bechdel

R makes a default choice about the number/width of bins:

ggplot(bechdel, aes(x = gross_2013)) + 
  geom_histogram()

You can override the default

Fewer, wider bins:

ggplot(bechdel, aes(x = gross_2013)) + 
  geom_histogram(bins = 10)

You can override the default

Many, thinner bins:

ggplot(bechdel, aes(x = gross_2013)) + 
  geom_histogram(bins = 100)

You can go too far either way

Oof.

ggplot(bechdel, aes(x = gross_2013)) + 
  geom_histogram(bins = 2)

Yikes.

ggplot(bechdel, aes(x = gross_2013)) + 
  geom_histogram(bins = 1500)

Part of the art here is developing an eye for what looks good.

Another option: instead of a histogram

ggplot(bechdel, 
       aes(x = gross_2013)) + 
  geom_histogram() + 
  labs(
    x = "2013 USD",
    title = "Film grosses"
  ) +
  theme_minimal(base_size = 20)

Another option: a density

ggplot(bechdel, 
       aes(x = gross_2013)) + 
  geom_density() + 
  labs(
    x = "2013 USD",
    title = "Film grosses (1990 - 2013)"
  ) +
  theme_minimal(base_size = 20)

Compare

Prettier. Smooths out the lumps and bumps. There are still defaults you could learn to override.

But let’s make it nice

What we have so far:

ggplot(bechdel, 
       aes(x = gross_2013)) + 
  geom_density() + 
  labs(
    x = "2013 USD",
    title = "Film grosses (1990 - 2013)"
  ) +
  theme_minimal(base_size = 20)

But let’s make it nice

We don’t need to see everything, do we?

ggplot(bechdel, 
       aes(x = gross_2013)) + 
  geom_density() + 
  labs(
    x = "2013 USD",
    title = "Film grosses (1990 - 2013)"
  ) +
  theme_minimal(base_size = 20) + 
  coord_cartesian(xlim = c(0, 1e9))

But let’s make it nice

A pop of color:

ggplot(bechdel, 
       aes(x = gross_2013)) + 
  geom_density(color = "firebrick") + 
  labs(
    x = "2013 USD",
    title = "Film grosses (1990 - 2013)"
  ) +
  theme_minimal(base_size = 20) + 
  coord_cartesian(xlim = c(0, 1e9))

But let’s make it nice

I can’t hear you:

ggplot(bechdel, 
       aes(x = gross_2013)) + 
  geom_density(color = "firebrick",
               linewidth = 2) + 
  labs(
    x = "2013 USD",
    title = "Film grosses (1990 - 2013)"
  ) +
  theme_minimal(base_size = 20) + 
  coord_cartesian(xlim = c(0, 1e9))

But let’s make it nice

Give it some body:

ggplot(bechdel, 
       aes(x = gross_2013)) + 
  geom_density(color = "firebrick",
               linewidth = 2,
               fill = "red") + 
  labs(
    x = "2013 USD",
    title = "Film grosses (1990 - 2013)"
  ) +
  theme_minimal(base_size = 20) + 
  coord_cartesian(xlim = c(0, 1e9))

But let’s make it nice

Too aggressive:

ggplot(bechdel, 
       aes(x = gross_2013)) + 
  geom_density(color = "firebrick",
               linewidth = 2,
               fill = "red",
               alpha = 0.5) + 
  labs(
    x = "2013 USD",
    title = "Film grosses (1990 - 2013)"
  ) +
  theme_minimal(base_size = 20) + 
  coord_cartesian(xlim = c(0, 1e9))

Yet another option: instead of a histogram

ggplot(bechdel, 
       aes(x = gross_2013)) + 
  geom_histogram() + 
  labs(
    x = "2013 USD",
    title = "Film grosses (1990 - 2013)"
  ) +
  theme_minimal(base_size = 20)

Yet another option: a boxplot

ggplot(bechdel, 
       aes(x = gross_2013)) + 
  geom_boxplot() + 
  labs(
    x = "2013 USD",
    title = "Film grosses (1990 - 2013)"
  ) +
  theme_minimal(base_size = 20)

How is a box plot drawn?

The middle of the box is the median. 50% of the data are below, and 50% are above:

How is a box plot drawn?

The lower edge of the box is the 25% quantile. 25% of the data are below, and 75% are above:

How is a box plot drawn?

The upper edge of the box is the 75% quantile. 75% of the data are below, and 25% are above:

Box plot facts

• A box plot is easier to read quickly than a histogram because it eliminates clutter and immediately draws your eye to those main features: center, spread, and skew;
• The box spans the middle 50% of the data;
• The width of the box is called the inner quartile range (IQR) and it measures spread;
• From the docs (?geom_boxplot):

The upper whisker extends from the hinge to the largest value no further than 1.5 * IQR from the hinge (where IQR is the inter-quartile range, or distance between the first and third quartiles). The lower whisker extends from the hinge to the smallest value at most 1.5 * IQR of the hinge. Data beyond the end of the whiskers are called “outlying” points and are plotted individually.

Box plots obscure modality

Same box plot:

Box plots obscure modality

Very different distributions:

So…now what?

Are we just eyeballing pictures?

No! There are numerical summaries you can compute to quantify your visual intuition:

• center: mean (aka average), median, mode, …
• spread: variance, standard deviation, inner quartile range (IQR), …

And there will be more: correlation coefficients, regression estimates, etc.

Some formulas (don’t sweat it)

Let \(x_1\), \(x_2\), …, \(x_n\) be the numbers listed in the column of our data frame. So \(n\) is the number of rows, or the sample size. Then:

The sample average (aka mean):

\[ \bar{x}=\frac{x_1+x_2+...+x_n}{n}=\frac{1}{n}\sum\limits_{i=1}^n x_i \]

The sample standard deviation:

\[ s = \sqrt{\frac{1}{n-1}\sum\limits_{i=1}^n(x_i-\bar{x})^2}. \]

Underneath the square root, you have the “average squared distance from the average,” or the sample variance.

How does the computer do it?

bechdel |>
  summarize(
    avg = mean(gross_2013, na.rm = TRUE),
    s = sd(gross_2013, na.rm = TRUE)
  )

# A tibble: 1 × 2
         avg          s
       <dbl>      <dbl>
1 271414482. 353676712.

• summarize creates a new data frame that stores the summaries;
• You can compute as many summaries as you want;
• To the left of the equal signs are your choice of column names in the new data frame you are creating. You can type whatever you want here (within reason);
• To the right of the equal signs is R code that computes the summaries. You must use the correct command names (case sensitive): mean, median, quantile, sd, var, etc;
• If you want to learn what these do, read the documentation (eg ?quantile).

Aside: the pipe operator |>

The pipe operator passes what comes before it into the function that comes after it as the first argument in that function:

sum(1, 2)

[1] 3

1 |> 
  sum(2)

[1] 3

We will use it to build up data summaries and transformations step-by-step.

Like a Russian stacking doll

More summaries

bechdel |>
  summarize(
    avg = mean(gross_2013, na.rm = TRUE),
    sdev = sd(gross_2013, na.rm = TRUE),
    q50 = median(gross_2013, na.rm = TRUE),
    q25 = quantile(gross_2013, 0.25, na.rm = TRUE),
    q75 = quantile(gross_2013, 0.75, na.rm = TRUE),
    iqr = IQR(gross_2013, na.rm = TRUE)
  )

# A tibble: 1 × 6
         avg       sdev        q50       q25       q75        iqr
       <dbl>      <dbl>      <dbl>     <dbl>     <dbl>      <dbl>
1 271414482. 353676712. 150218382. 53689120. 354101229 300412109.

Numbers and pictures, working together

Where are the grosses concentrated?

If there is only one peak, then…

The mean is more sensitive to “outliers” than the median. The mean will get dragged in the direction of the skew more aggresively.

Summary

Visualizing and summarizing one numerical variable:

bechdel |>
  summarize(
    avg = mean(gross_2013, na.rm = TRUE),
    sdev = sd(gross_2013, na.rm = TRUE),
    q50 = median(gross_2013, na.rm = TRUE),
    q25 = quantile(gross_2013, 0.25, na.rm = TRUE),
    q75 = quantile(gross_2013, 0.75, na.rm = TRUE),
    iqr = IQR(gross_2013, na.rm = TRUE)
  ) |>
  glimpse()

Rows: 1
Columns: 6
$ avg  <dbl> 271414482
$ sdev <dbl> 353676712
$ q50  <dbl> 150218382
$ q25  <dbl> 53689120
$ q75  <dbl> 354101229
$ iqr  <dbl> 300412109

Exploring a single categorical variable

The test results

The test result for the films is a categorical variable:

bechdel$clean_test

 [1] "notalk" "ok"     "notalk" "notalk" "men"    "men"    "notalk" "ok"    
 [9] "ok"     "notalk"

Visualization and summarization will be a bit more straightforward here. Things get fun when categorical variables interact with other stuff.

Picture: bar plot

ggplot(bechdel, aes(y = clean_test)) + 
  geom_bar()

Picture: bar plot

ggplot(bechdel, aes(x = clean_test)) + 
  geom_bar()

Numerical summary: tally it up

bechdel |>
  count(clean_test)

# A tibble: 5 × 2
  clean_test     n
  <chr>      <int>
1 dubious      124
2 men          168
3 notalk       450
4 nowomen      120
5 ok           753

Numerical summary: proportions

bechdel |>
  count(clean_test) |>
  mutate(
    prop = n / sum(n)
  ) |>
  arrange(prop)

# A tibble: 5 × 3
  clean_test     n   prop
  <chr>      <int>  <dbl>
1 nowomen      120 0.0743
2 dubious      124 0.0768
3 men          168 0.104 
4 notalk       450 0.279 
5 ok           753 0.466 

Seriously, it’s just like the dolls

This is so much harder to read

Exploring two numerical variables

Go from one column of numbers to two

bechdel |>
  select(budget_2013, gross_2013)

# A tibble: 1,615 × 2
   budget_2013 gross_2013
         <dbl>      <dbl>
 1    13000000   67878146
 2    45658735   55078343
 3    20000000  211714070
 4    61000000  208105475
 5    40000000  190040426
 6   225000000  184166317
 7    92000000  371598396
 8    12000000  102648667
 9    13000000   36014634
10   130000000  304895295
# ℹ 1,605 more rows

Scatter plot

ggplot(bechdel, 
       aes(x = budget_2013, 
           y = gross_2013)) + 
  geom_point() + 
  labs(
    x = "Budget (2013 USD)",
    y = "Gross (2013 USD)",
    title = "Film finances (1990 - 2013)"
  ) +
  theme_minimal(base_size = 20)

Add a “trend” line

ggplot(bechdel, 
       aes(x = budget_2013, 
           y = gross_2013)) + 
  geom_point() + 
  geom_smooth() + 
  labs(
    x = "Budget (2013 USD)",
    y = "Gross (2013 USD)",
    title = "Film finances (1990 - 2013)"
  ) +
  theme_minimal(base_size = 20)

Add a “trend” line

ggplot(bechdel, 
       aes(x = budget_2013, 
           y = gross_2013)) + 
  geom_point() + 
  geom_smooth(method = "lm") + 
  labs(
    x = "Budget (2013 USD)",
    y = "Gross (2013 USD)",
    title = "Film finances (1990 - 2013)"
  ) +
  theme_minimal(base_size = 20)

A numerical summary: correlation

bechdel |>
  summarize(
    r = cor(gross_2013, budget_2013, use = "complete.obs")
  )

# A tibble: 1 × 1
      r
  <dbl>
1 0.687

Correlation (more on this next week)

• Measures the strength and direction of the linear association between two numerical variables;
• Tells you how tightly the points cluster around a straight line;
• Ranges between -1 and 1;
• Same sign as the slope.

Exploring one categorical variable and one numerical variable together

Back to the research question

What does passing The Bechdel Test mean for a film’s finances?

• Categorical variables: clean_test and binary;
• Numerical variable: roi.

How does roi change depending on the category? To answer, we can use all the tools for univariate numerical data, but within the groups.

Side-by-side box plots (preferred)

ggplot(bechdel, aes(x = roi, y = clean_test, color = binary)) +
  geom_boxplot() +
  labs(
    title = "Return on investment vs. Bechdel test result",
    x = "Return-on-investment (gross / budget)",
    y = "Detailed Bechdel result",
    color = "Bechdel\nresult"
  ) +
  coord_cartesian(xlim = c(0, 16))

Side-by-side histograms (…eh)

bechdel |>
  mutate(
    clean_test = fct_rev(clean_test)
  ) |>
  ggplot(aes(x = roi, fill = binary)) +
  geom_histogram(binwidth = 0.5) +
  facet_wrap(~clean_test, nrow = 5) + 
  labs(
    title = "Return on investment vs. Bechdel test result",
    x = "Return-on-investment (gross / budget)",
    y = "Detailed Bechdel result",
    fill = "Bechdel\nresult"
  ) +
  coord_cartesian(xlim = c(0, 16))

Within-group summaries

bechdel |>
  group_by(clean_test) |>
  summarize(
    avg = mean(roi, na.rm = TRUE),
    q50 = median(roi, , na.rm = TRUE)
  )

# A tibble: 5 × 3
  clean_test   avg   q50
  <chr>      <dbl> <dbl>
1 dubious    11.2   3.80
2 men         9.30  3.96
3 notalk      6.93  3.69
4 nowomen     9.52  3.27
5 ok          7.99  4.21

The conclusion depends on the summary you choose. Which is more appropriate?

Wrap-up

The art

Today you saw some basic building blocks. The art of data visualization is in their combination. How do you trick the human brain into appreciating complex multivariate relationships when the human brain cannot really visualize in high dimensions?

Computational themes

Every coding task in this class will be some combo of these two things:

Building cakes (ggplot)

Stacking dolls (pipe |>)

Master these, and everything will be coming up roses!