a cognitive psychologist is interested in whether completing a six-week speed-reading course affects reading comprehension. she has students complete a reading comprehension task before and after a six-week speed-reading course. the cognitive psychologist randomly selects a sample of 42 students who, at the beginning of the study, scored an average of 107 points on the reading comprehension task. after the speed-reading course, the students scored an average of 4 points lower. the standard deviation of the difference scores was 12. since the sample size is larger than 30, the cognitive psychologist can assume that the sampling distribution of md is normal.