Example 3 in this section illustrated the procedure for finding a prediction interval for an individual value of y. When using a specific value x0 for predicting the mean of all values of y, the confidence interval is as follows:

where

The critical value t_?/2 is found with n - 2 degrees of freedom. Use the pizza subway data from the Chapter Problem to find a 95% confidence interval estimate of the mean subway fare given that the price of a slice of pizza is $2.25.

- E < < + E E = ta/2 n + n(x0 - 7) n(x3) (x)2