Regression: Identifying Good and Bad Leverage Points

When dealing with regression, a well known concern is that a few bad leverage points can result in a poor fit to the bulk of the data. This is the case even when using various robust estimators, which is known as contamination bias. Currently, a relatively e ective method for detecting bad leverage points is based in part on the least median of squares regression estimator. This note suggests a modification of this method that is better able to detect bad leverage points. The modification also provides a substantially better technique for dealing with contamination bias.