Page 239 - Statistics II for Dummies

Chapter 13: Forming Associations with Two-Way Tables 223

Table 13-1 Two-Way Table for the Cellphone Data
Personal Calls: Yes Personal Calls: No
Males
Females

Notice that Table 13-1 has four empty cells inside of it (not counting the
empty space in the upper-left corner). Because gender has two choices
(male or female) and personal cellphone use has two choices (yes or no), the
resulting two-way table has 2 * 2 = 4 cells.
To figure out the number of cells in any two-way table, multiply the number of
possible categories for the row variable times the number of possible catego-
ries for the column variable.

Filling in the cell counts

After you set up the table with the appropriate number of rows and columns,
you need to fill in the appropriate numbers in each of the cells of the two-way
table. The number in each cell of a two-way table is called the cell count for
that cell. Of the four cells in the two-way table shown in Table 13-1, the upper-
left cell represents the number of males who use their cellphones for per-
sonal calls. With the information you have in the cellphone example, the cell
count for this cell is 325. You also know that 427 females use their cellphones
for personal calls, and this number goes into the lower-left cell.
To figure out the numbers in the remaining two cells, you do a bit of sub-
traction. You know from the information given that the total number of male
cellphone users in the survey is 508. Each male either uses his cellphone for
personal calls (falling into the yes group) or doesn’t use it for personal calls
(falling into the no group). Because 325 males fall into the yes group, and you
have 508 males total, 183 males (508 – 325 = 183) don’t use their cellphones
for personal calls. This number is the cell count for the upper-right cell of
the two-way table. Finally, because 508 females took the survey, and 427 of
them use their cellphones for personal calls, you know that the rest of them
(508 – 427 = 81) don’t. Therefore, 81 is the cell count for the lower-right cell
of the table. Table 13-2 shows the completed table for the cellphone user
example, with the four cell counts filled in.

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