In the Bivariate platform, use the Fit Special option to fit regression models with transformed variables. You can also constrain the slope and intercept, fit a polynomial model of specific degree, or center the polynomial.
The Fit Special option opens the Specify Transformation or Constraint window that contains the following options:
Y or X Transformation
Enables you to specify a transformation for the Y or X variable. The following transformations are available: natural logarithm, square root, square, reciprocal, and exponential.
Degree
Enables you to fit a polynomial of a specified degree.
Centered Polynomial
Enables you to center the polynomial. Deselect or select to disable or enable polynomial centering. Centering polynomials stabilizes the regression coefficients and reduces multicollinearity.
Note: Polynomial centering is not supported for transformations of the X variable.
Constrain Intercept to
Enables you to constrain the model intercept to a specified value.
Constrain Slope to
Enables you to constrain the model slope to a specified value.
Tip: To add the Y = X line to your plot select Fit Special and then select Constrain Intercept to: 0 and Constrain Slope to: 1.
For more information about the options in the Transformed Fit menu, see Bivariate Fit Options. For an example, see Example of the Fit Special Option.