As it has been standardized, the response z vmail messages has a standard deviation of 1.0. What would be the typical error in predicting z vmail messages if we simply used the sample mean response and no information about day calls? Now, from the printout, what is the typical error in predicting z vmail messages, given z day calls? Comment. For Exercises 31–38, use the output from the regression of an unspecified y on an unspecified x in Table 8.18 to answer the questions.