1. Analyzing One-Variable Data
Lesson 1.2
Displaying Categorical Data
Statistics and Probability with Applications, 3rd Edition
Starnes, Tabor
Bedford Freeman Worth Publishers

2. Displaying Categorical Data
Learning Targets
After this lesson, you should be able to:
Make and interpret bar charts of categorical data.
Interpret pie charts.
Identify what makes some graphs of categorical data deceptive.

3. Displaying Categorical Data
A frequency table or relative frequency table summarizes a variable’s distribution in numerical form. To display the distribution more clearly, use a graph.
Bar Chart
A bar chart shows each category as a bar. The heights of the bars show the category frequencies or relative frequencies.
Pie Chart
A pie chart shows each category as a slice of “pie.” The areas of the slices are proportional to the category frequencies or relative frequencies.

4. Displaying Categorical Data
How to Make a Bar Chart
Draw and label the axes. Put the name of the categorical variable under the horizontal axis. To the left of the vertical axis, indicate if the graph shows the frequency (count) or relative frequency (percent or proportion) of individuals in each category.
“Scale” the axes. Write the names of the categories at equally spaced intervals under the horizontal axis. On the vertical axis, start at 0 and place tick marks at equal intervals until you exceed the largest frequency or relative frequency in any category.
Draw bars above the category names. Make the bars equal in width and leave gaps between them. Be sure that the height of each bar corresponds to the frequency or relative frequency of individuals in that category.

5. Displaying Categorical Data
Level of Education
Percent
Less than high school
13.2
High school graduate
22.6
Some college
28.7
Bachelor’s degree
24.9
Advanced degree
10.6

6. Displaying Categorical Data
You can use a pie chart when you want to emphasize each category’s relation to the whole.
Level of Education
Percent
Less than high school
13.2
High school graduate
22.6
Some college
28.7
Bachelor’s degree
24.9
Advanced degree
10.6

7. Displaying Categorical Data
Bar charts and pie charts can display a distribution of categorical data. A side by side bar chart can also be used to compare the distribution of a categorical variable in two or more groups. It’s a good idea to use relative frequencies (percents or proportions) when comparing, especially if the groups have different sizes.

8. Displaying Categorical Data
Bar charts are a bit dull to look at. It is tempting to replace the bars with pictures or to use special 3-D effects to make the graphs seem more interesting. Don’t do it! Our eyes react to the area of the bars as well as to their height. When all bars have the same width, the area varies in proportion to the height, and our eyes receive the right impression about the quantities being compared.
Preferred Party
Frequency
Donut 5
Pasta 18
Pizza 7
Total 30

9. Which cell phone speaks to you?
LESSON APP 1.2
Which cell phone speaks to you?
The Pew Research Center asked a random sample of 2024 adult cell-phone owners from the United States which type of cell phone they own: iPhone, Android, or other (including non-smartphones).
The frequency table displays the results.
The side-by-side bar chart displays the distribution of phone ownership for each of three age groups.
Type of cell phone
Frequency
iPhone
467
Android
503
Other
1054
Total
2024
Make a bar chart to display the distribution of phone ownership among all 2024 people in the sample. Describe what you see.
Write a few sentences comparing the distributions of phone ownership for the three age groups.

10. Displaying Categorical Data
Learning Targets
After this lesson, you should be able to:
Make and interpret bar charts of categorical data.
Interpret pie charts.
Identify what makes some graphs of categorical data deceptive.