Adding a Layer Variable

You can add a layer variable to create a three-way table in which categories of the row and column variables are further subdivided by categories of the layer variable. This variable is sometimes referred to as the control variable because it may reveal how the relationship between the row and column variables changes when you "control" for the effects of the third variable.

1. Open the Crosstabs dialog box again.
2. Click Cells.
3. Deselect (clear) Row Percents.
4. Click Continue.
5. Select Level of Education (ed) as the layer variable.
6. Click OK to run the procedure.

If you look at the crosstabulation table, it might appear that the only thing we have accomplished is to make the table larger and harder to interpret. But if you look at the table of chi-square statistics, you can easily see that in all but one of the education categories, the apparent relationship between income and PDA ownership disappears (typically, a significance value less than 0.05 is considered "significant").

This suggests that the apparent relationship between income and PDA ownership is merely an artifact of the underlying relationship between education level and PDA ownership. Since income tends to rise as education rises, apparent relationships between income and other variables may actually be the result of differences in education.