Page 240 - Statistics II for Dummies

224 Part IV: Building Strong Connections with Chi-Square Tests

Table 13-2 Completed Two-Way Table for the Cellphone Data
Personal Calls: Yes Personal Calls: No
Males 325 183 = (508 – 325)
Females 427 81 = (508 – 427)

Just to save you a little time, if you have the total number in a group and the
number of individuals who fall into one of the categories of the two-way table,
you can determine the number falling into the remaining category by subtract-
ing the total number in the group minus the number in the given category. You
can complete this process for each remaining group in the table.

Making marginal totals

One of the most important characteristics of a two-way table is that it gives
you easy access to all the pertinent totals. Because every two-way table is
made up of rows and columns, you can imagine that the totals for each row
and the totals for each column are important. Also, the grand total is impor-
tant to know.

If you take a single row and add up all the cell counts in the cells of that row,
you get a marginal row total for that row. Where does this marginal row total
go on the table? You guessed it — out in the margin at the end of that row.
You can find the marginal row totals for every row in the table and put them
into the margins at the end of the rows. This group of marginal row totals for
each row represents what statisticians call the marginal distribution for the
row variable.

The marginal row totals should add up to the grand total, which is the total
number of individuals in the study. (The individuals may be people, cities,
dogs, companies, and so on, depending on the scenario of the problem at
hand.)

Similarly, if you take a single column and add up all the cell counts in the
cells of that column, you get the marginal column total for that column. This
number goes in the margin at the bottom of the column. Follow this pattern
for each column in the table, and you have the marginal distribution for the
column variable. Again, the sum of all the marginal column totals equals the
grand total. The grand total is always located in the lower-right corner of the
two-way table.

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