Leverage values are employed to identify outliers with
respect to their x values. This value is a measure of the
distance between the observation’s x values and the centre of
the data. If the leverage value of an observation is large, the
observation is outlying with respect to its x values. The
diagonal elements of the hat matrix (called leverage values)
are a useful indicator of whether or not an observation is
outlying with respect to its x values. The leverage value, , ii h
for the ith observation in the data matrix X is given by