9.39 In a study of academic procrastination, the authors of the paper “Correlates and Consequences of Behavioral Procrastination” (Procrastination, Current Issues and New Directions [2000]) reported that for a sample of 411 undergraduate students at a midsize public university preparing for a final exam in an introductory psychology course, the mean time spent studying for the exam was 7.74 hours and the standard deviation of study times was 3.40 hours. For purposes of this exercise, assume that it is reasonable to regard this sample as representative of students taking introductory psychology at this university.

a. Construct a 95% confidence interval to estimate m, the mean time spent studying for the final exam for students taking introductory psychology at this university. (Hint: See Example 9.8.)

b. The paper also gave the following sample statistics for the percentage of study time that occurred in the 24 hours prior to the exam:

n 5 411 x 5 43.18 s 5 21.46 Construct and interpret a 90% confidence interval for the mean percentage of study time that occurs in the 24 hours prior to the exam.