The Economy
Growth example uses the Distance by Distance graph, which is generated
using the ROBUSTREG procedure. The main purpose of robust regression
is to detect outliers and leverage points and to provide stable results
in the presence of such influential points.
This graph
identifies leverage points for economic growth data for different
countries. The distances displayed are appropriately scaled. They
measure the distance from the center of the data to the observation.
The vertical axis distance is a robust version of the classical distance
measure on the horizontal axis. Observations with large discrepancies
between these two distances have high leverage. The diagonal reference
line helps to identify these observations. Such observations also
have large ROBUST MCD distance; those observations that are above
the horizontal leverage cutoff line are considered leverage points.
The data
used in this example was obtained from a national growth study conducted
by De Long and Summers in 1991.