Identifying influential points

  1. To check Cook's distance by the centered leverage value, recall the Chart Builder.
    Figure 1. Chart Builder
    Chart Builder
  2. Select Cook's Distance as the y variable.
  3. Select Centered Leverage Value as the x variable.
  4. Click OK.
Figure 2. Cook's distance by centered leverage value
Scatterplot with Cook's distance on the vertical axis and centered leverage values on the horizontal axis. Most values are in a cluster in the lower left corner near the origin. 3000GT, Metro, and Viper are all far outside main cluster of values.

The resulting scatterplot shows a few unusual points. The 3000GT has a large Cook's distance, but it does not have a high leverage value, so while it adds a lot of variability to the regression estimates, it likely did not affect the slope of the regression equation. The Viper has a high leverage value, but does not have a large Cook's distance, so it is not likely to have exerted undue influence on the model. The most worrisome case is the Metro, which has both a high leverage and a large Cook's distance. The next step would be to run the analysis without this case, but we will not pursue this here.

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