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Publication date: 11/10/2021

Fit Special

Using the Fit Special option, you can transform Y and X. Although data can be transformed for various reasons, it is often done to render data more plausibly normal, so that the appropriate tests can then be conducted. You can also constrain the slope and intercept, fit a polynomial of specific degree, and center the polynomial.

Note: For an example of this option, see Example of the Fit Special Option.

The Specify Transformation or Constraint Window contains the following options:

Y or X Transformation

Use one of these options to transform the Y or X variable:

Natural logarithm

Square root





Use this option to fit a polynomial of the specified degree.

Centered Polynomial

To turn off polynomial centering, deselect the Centered Polynomial check box (Figure 5.19). Note that for transformations of the X variable, polynomial centering is not performed. Centering polynomials stabilizes the regression coefficients and reduces multicollinearity.

Constrain Intercept to

Select this check box to constrain the model intercept to be the specified value.

Constrain Slope to

Select this check box to constrain the model slope to be the specified value.

For more information about the red triangle options for this fit, see Fitting Menus.

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