If the ith observation is outlying in terms of its x values
and therefore has a large value of ii h , it exercises
substantial weight (leverage) in determining the fitted value
ˆ . i y A leverage value ii h is usually considered to be large,
if it is more than twice as large as the mean leverage value.
That is, leverage values greater than 2(k +1) n are
considered by this rule to indicate outlying observations
with regard to their x values.