A professor studied a sample of 112 students and found a positive association

between Students' study effort (X) and their grades (Y). She decides to introduce a

control variable, Student's IQ (Z), and obtained the following first-order table:

Student's IQ level (Z) Low High Study effort (X) Study effort (X) Grades (Y) Low High All Low High All Low 16 12 28 19 7 26 High 10 16 26 9 23 32 Total 26 28 54 28 30 58