Set Delta for Power

This option specifies the difference in the mean response that you want to be able to detect for model effects. Power calculations appear in the Power Analysis outline within the Design Evaluation outline. Power is calculated for each model parameter based on detecting the specified difference of delta. For categorical effects, the power calculation is based on detecting a maximum change of delta between any two levels.

For example, suppose that you want to detect a change of 3 units in the mean response. All of your factors are continuous. Because your factors are expressed in coded units (the coded levels are -1 and 1), a change of 3 units in the response corresponds to parameter coefficient values of 1.5. When you specify 3 in the Set Delta for Power text box, the Anticipated Coefficients in the Power Analysis outline are set to 1.5. For each parameter, the probability of detecting the change of 3 units in the response appears in the Power column to the right of the parameter.

By default, delta is set to 2. The default coefficient for each continuous effect is set to 1. An n-level categorical factor is represented by n–1 indicator variables. The default coefficients for the n–1 terms (which represent the categorical factor) are alternating values of 1 and -1. The default coefficients for an interaction effect with more than one degree of freedom are also alternating values of 1 and -1.

Note: The order in which parameters appear in the Power Analysis report might not be identical to their order in the Parameter Estimates report obtained using Standard Least Squares. This difference can occur only when the model contains an interaction with more than one degree of freedom.

Given a specified value of delta, each coefficient in the Anticipated Coefficients list is set at delta/2 multiplied by the default coefficient. For a continuous factor, this assignment ensures that a difference of delta is detected with the calculated power. For a categorical factor, this assignment of coefficients ensures that a maximum difference of delta between any two levels is detected with the calculated power.