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Chapter 19: Two-Way Tables and Independence
don’t apply to all 100 campers surveyed; however, if you tried to examine the
raw data from all 100 rows of this data set by hand, you wouldn’t make much
progress in seeing patterns without a lot of hard work.
In order to get a handle on what’s happening in a large data set when you are
examining two categorical variables, you organize your data into a two-way
table. The following sections take you through it.
Setting up the cells
A two-way table organizes categorical data from two variables by using rows
to represent one variable (such as pet camping — yes or no) and columns to
represent the other variable (such as opinion on a pet section — support or
oppose). Each person appears exactly once in the table.
Continuing with the camping example I start earlier in this chapter, in Table 19-2
I summarize the results from all 100 campers surveyed. 297
Table 19-2 Two-Way Table of Pet Camping Survey Data
(All 100 Rows)
Support Separate Oppose Separate
Pet Section Pet Section
Pet Camper 20 10
Non–Pet Camper 55 15
Table 19-2 has 2 ∗ 2 = 4 numbers in it. These numbers represent the cells of
the two-way table; each one represents an intersection of a row and column.
The cell in the upper left corner of the table represents the 20 people who are
pet campers supporting a pet section. In the upper right cell 10 people are pet
campers opposing a pet section. In the lower left are the 55 non–pet campers
who want a pet section; the 15 people in the lower right are non–pet campers
opposing a pet section.

Figuring the totals

Before getting to the nitty-gritty analysis of a two-way table in the later section
“Interpreting Results from a Two-Way Table,” you calculate some totals and
add them to the table for later reference. You summarize each variable sepa-
rately by calculating the marginal totals, which represent the total number in
each row (for the first variable) and the total number in each column (for the

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