Refer to the Acoustical Science & Technology (Vol. 35, 2014) study of irrelevant speech effects, Exercise 2.34 (p. 49). Recall that subjects performed a memorization task under two conditions: (1) with irrelevant background speech and (2) in silence. The difference
in the error rates for the two conditions-called the relative difference in error rate (RDER)-was computed for each subject. Descriptive statistics for the RDER values
are shown in the SAS printout.

a. Based on the histogram shown in Exercise 2.34, which rule is more appropriate for describing the distribution of the RDER values? Why?
b. Apply the rule you selected in part a to the data. Specifically, estimate the proportion of RDER values that fall in the interval x̅ ± 2s