Residuals vs. Fit Values Plots

  1. To produce a scatterplot of residuals by fit values for the Linear model, from the menus choose:

    Graphs > Chart Builder...

    Figure 1. Chart Builder
    Chart Builder
  2. Select the Scatter/Dot gallery and choose Simple Scatter.
  3. Select Error for sales with advert from CURVEFIT, MOD_1 LINEAR as the y variable and Fit for sales with advert from CURVEFIT, MOD_1 LINEAR as the x variable.
  4. Click OK.
    Figure 2. Scatterplot of residuals by fit values for linear modell
    Scatterplot of residuals by fit values for linear modell

    This plot reinforces your suspicions from the curve fit plot. There is a clear "inverted U" shape to the points, which means that there is a pattern in the data that is not captured by the Linear model.

    Figure 3. Chart Builder
    Chart Builder
  5. To produce a scatterplot of residuals by fit values for the Quadratic model, recall the Chart Builder.
  6. Click Reset to restore the default settings.
  7. Select the Scatter/Dot gallery and choose Simple Scatter.
  8. Select Error for sales with advert from CURVEFIT, MOD_1 QUADRATIC as the y variable and Fit for sales with advert from CURVEFIT, MOD_1 QUADRATIC as the x variable.
  9. Click OK.
Figure 4. Scatterplot of residuals by fit values for quadratic modell
Scatterplot of residuals by fit values for quadratic modell

These residuals do not show a pattern, thus the Quadratic model is acceptable in the sense the residuals are independent of the fit values.

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