modified from Stock and Watson Exercise 4.5) A professor decides to run an experiment to measure the effect of time pressure on final exam scores. She gives each of the 500 students in her course the same final exam, but some students have 45 minutes to complete the exam while others have 60 minutes. Each student is randomly assigned one of the examination times based on the flip of a coin. Let Yi denote the number of points (out of 100) scored on the exam by the ith student (i = 1, . . . , 500), and let Xi be the amount of minutes that the student had to complete the exam (so Xi = 45 or Xi = 60). Consider the linear regression model Yi = 0 + 1Xi + ui. (a) Explain what the term ui represents here. Why will different students have different values of ui