Chapter 3: Displaying and Describing Categorical Data*Data Analysis

*Frequency Tables, Bar Charts, Pie Charts Contingency Tables

Titanic Who: People on the Titanic

What: survival status, age, sex, ticket class

When: April 14, 1912

Where: North Atlantic

How: A variety of sources and internet sites

Why: Historical interest

Data Analysis Rule #1: Make a picture

a display of your data will revewal thins you are not likely to see in a table of numbers and will help you THINK clearly about the patterns and relationships that may be hiding in your data

Rule # 2: Make a picture A well-designed display will SHOW the important features

and patterns in your data. A picture will also show you the things you did not expect to see: the extraordinary (possibly wrong) data values or unexpected patterns

Rule # 3: Make a picture The best way to TELL others about our data is with a well-

chosen picture.

Picking a Display Determine if your variable is categorical or quantitative

If categorical: frequency tables bar charts pie charts contingency tables

If quantitative: wait till next chapter!

Frequency Tables records the totals and the category names

Example:

Class Count

First 325

Second 285

Third 706

Crew 885

Relative Frequency Table

displays the percentages and category names

Example:

Class %

First 14.77

Second 12.95

Third 32.08

Crew 40.21

Proportion: the fraction of data in each category

(divide the counts by the total number of cases)

Relative = Percent

Bar Charts Displays the distribution of a categorical variable,

showing the counts for each category next to each other for easy comparison.

Bars should be the same width Bars should have spaces between them Axis should both be labeled with a title and/or scale

Pie Charts shows the whole group of cases as a circle. They slice the

circle into pieces whose size is proportional to the fractions of the whole in each category.

Categorical Data Condition The data are counts or percentages of individuals in

categories.

Before you make a pie chart or a bar graph you must check this condition!!!