Psychology 2211 Assignment 3

1 Sixteen subjects were randomly assigned to one of two groups in an experiment investigating the effect of stress on test-taking performance. Subjects in group 1 completed a 100-item test with no time limit, while subjects in group 2 were allowed only 30 seconds to complete each item on the same test. The following data were obtained:

Group 1 Group 2

86 72

72 57

78 65

74 72

83 86

72 48

81 54

74 41

X-bar₁ = 77.5 X-bar₂ = 61.875

s₁ = 5.35 s₂ = 14.69

Is there any significant difference between the performance of the two groups? (a .05)

2. The same experiment was now repeated with a sample of 25 subjects. Again, group 1 did the test with no time limit and group 2 was timed, yielding the following data:

Group 1 Group 2

72 64

84 85

81 58

76 76

79 83

83 56

83 92

72 81

81 72

80 60

83 64

80 88

___ 55

n₁ = 12 n₂ = 13

X-bar1= 79.5 X-bar₂ = 71.85

s₁ = 4.123 s₂ = 13.088

Did subjects in group 2 score lower, on average, than subjects in group 1? (a .01)

3. A new pollution control device for automobiles is claimed not only to reduce carbon monoxide emission but also to increase a car's miles per gallon. To test the latter claim, the mileage per gallon of 10 randomly selected cars is measured first without and then with the device, yielding the following data

Miles per gallon

Car Without Device With Device

1 28 32

2 20 21

3 44 36

4 7 10

5 20 20

6 11 14

7 11 9

8 34 32

9 9 14

10 48 46

Does the device significantly increase cars' mileage per gallon ( .05)?

Answers:

1. F_obt = 7.54, reject H₀;, do Wilcoxon; T_obt = 90.5, reject H₀

2. F_obt = 10.08, reject H_0, do Wilcoxon; Z_obt = 1.197, do not reject H₀

3. t = .163, Do not reject H₀; no evidence that the device increases cars' mpg