The diagonal elements of the hat matrix are often used to denote leverage—that is, a point that is unusual in its location in the x-space and that may be influential. Generally, the ith point is called a leverage point if its hat diagonal hii exceeds 2p/n, which is twice the average size of all the hat diagonals. Recall that p k 1.

(a) Table 12-12 contains the hat diagonal for the wire bond pull strength data used in Example 12-1.

Find the average size of these elements.

(b) Based on the criterion above, are there any observations that are leverage points in the data set?