To see what difference class attendance made, a professor sampled grades from his large statistics class of

530 students. From the 220 students who attended class less than half the time (the "irregulars"), he took

a random sample of 5 grades. From the remaining 310 students who attended at least half the time (the

"regulars"), he took an independent random sample of 5 other grades:

Irregulars

Regulars

41

69

81

56

52

83

69

70

62

92

Note that, mean difference is simply the mean of the difference. So you take the difference for each value in

each group and that is your new variable. From there, you simply take the mean of the differences (hence

the term mean difference) to calculate your confidence interval.

(a)

(5 pts) Construct a 95% confidence interval for the mean difference between the two groups of students?

(b)

(5 pts) To what extent does this support the contention that "it is worth 13 marks to come to class

regularly?"