The Variables Tab

You can use the Variables tab to specify the response and explanatory variables for the TPSPLINE MODEL statement. The Y variable specifies the dependent (response) variable, and the X variable specifies the independent (smoothing) variable. The Thin-Plate Spline analysis supports a single dependent variable and a single smoothing variable. Semiparametric fits are not supported: if your data have a polynomial trend, you should subtract the trend and use thin-plate splines to model the residuals.