Interaction Plot
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Stat > ANOVA > Interaction Plot

Interaction Plot creates a single interaction plot for two factors, or a matrix of interaction plots for three to nine factors. An interaction plot is a plot of means for each level of a factor with the level of a second factor held constant. Interaction plots are useful for judging the presence of interaction.

Interaction is present when the response at a factor level depends upon the level(s) of other factors. Parallel lines in an interaction plot indicate no interaction. The greater the departure of the lines from the parallel state, the higher the degree of interaction. To use interaction plot, data must be available from all combinations of levels.

This dialog box generates a plot that uses data means. After you have fit a model, use the stored model to generate plots that use fitted means.

Data means are the raw response variable means for each factor level combination whereas fitted means use least squares to predict the mean response values of a balanced design. Therefore, the two types of means are identical for balanced designs but can be different for unbalanced designs. While you can use raw data with unbalanced designs to obtain a general idea of which main effects may be evident, it is generally good practice to use the fitted means to obtain more precise results.

Use Interaction plots for factorial designs to generate interaction plots specifically for 2-level factorial designs, such as those generated by Fractional Factorial Design, Central Composite Design, and Box-Behnken Design.

Dialog box items

Responses: Enter the columns containing the response data.

Factors: Enter the columns containing the factor levels.

Display full interaction plot matrix: Check to display the full interaction matrix when more than two factors are specified instead of displaying only the upper right portion of the matrix. In the full matrix, the transpose of each plot in the upper right displays in the lower left portion of the matrix. The full matrix takes longer to display than the half matrix.

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