A company offers a seminar on a technical subject to its employees. Before the seminar begins, each employee scores their self-rating of knowledge on the subject. Three months after the seminar, each employee again rates their knowledge of the subject. The data are shown in Table 5.18. 

(a) Is there evidence that employees believe their knowledge has improved? 

(b) There were actually 12 employees in the seminar, but 6 did not return the follow-up rating. What type of bias might this nonresponse introduce into the analysis?

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Table 5.18 Data for Exercise 10. Employee 1 2 3 4 5 6 Before Seminar 12 16 After Seminar 15 16 115 16 25 10 17 15 18 78 12 14 21 15 17 57