Entering two-way ANOVA data

Groups are defined by rows and columns

Prism organizes data for two-way ANOVA differently than do most other programs.

Prism does not use grouping variables. Instead, use rows and columns to designate the different groups (levels) of each factor. Each data set (column) represents a different level of one factor, and each row represents a different level of the other factor.

From the Welcome or New table dialog, choose the Grouped tab to set up the right kind of table.

Two ways to enter the data

Before entering data, choose which grouping variable is defined by the rows, and which is defined by the columns. For example, if you are comparing men and women at three time points, there are two ways to organize the data:

Of course, the ANOVA results will be identical no matter which way you enter the data. But the choice does matter, as it influences how the graph will appear and how Prism can do multiple comparison post tests.

Your choice affects how the graph will look

When you create a graph of the data, the points or bars from each column can have a different appearance and color.

If you enter data as shown in the first approach above, men and women will appear in bars of different color, with three bars of each color representing the three time points. This is illustrated in the left panel below.

The right panel below shows a graph of the second data table shown above. There is one bar color and fill for Before, another for During, and another for After. Men and Women appear as two bars of identical appearance.

Prism's multiple comparison post tests only compare within a row

Prism can only perform post tests within a row, comparing columns. Using the first approach, Prism compares Men vs. Women at each time point. Using the second approach, Prism compares Before vs. During, Before vs. After, and During vs. After for each gender. Arrange your data so these post tests address your scientific questions.

Use the transpose analysis to change your mind

What happens if after entering and analyzing your data using one of the choices above, you then realize you wish you had done it the other way? You don't need to reenter your data. Instead use Prism's transpose analysis, and then do two-way ANOVA on the results.