Test for the Equality of k Population Means

1. Test for the Equality of k Population Means • Hypotheses H0: 1=2=3=. . . = k Ha: Not all population means are equal F = MSTR/MSE • Test Statistic • Rejection Rule p-value Approach: Reject H0 if p-value <a Critical Value Approach: Reject H0 if F>Fa where the value of F is based on an F distribution with k - 1 numerator d.f. and nT - k denominator d.f.

2. Test for the Equality of k Population Means

3. Testing for the Equality of k Population Means: A Completely Randomized Experimental Design • Example: AutoShine, Inc. AutoShine, Inc. is considering marketing a long- lasting car wax. Three different waxes (Type 1, Type 2, and Type 3) have been developed. In order to test the durability of these waxes, 5 new cars were waxed with Type 1, 5 with Type 2, and 5 with Type 3. Each car was then repeatedly run through an automatic carwash until the wax coating showed signs of deterioration.

4. Testing for the Equality of k Population Means: A Completely Randomized Experimental Design • Example: AutoShine, Inc. The number of times each car went through the carwash before its wax deteriorated is shown on the next slide. AutoShine, Inc. must decide which wax to market. Are the three waxes equally effective? Factor . . . Car wax Treatments . . . Type I, Type 2, Type 3 Experimental units . . . Cars Response variable . . . Number of washes

5. Testing for the Equality of k Population Means: A Completely Randomized Experimental Design Wax Type 1 Wax Type 2 Wax Type 3 Observation 1 2 3 4 5 27 30 29 28 31 33 28 31 30 30 29 28 30 32 31 Sample Mean 29.0 30.4 30.0 Sample Variance 2.5 3.3 2.5

6. Testing for the Equality of k Population Means: A Completely Randomized Experimental Design • Hypotheses H0: 1=2=3 Ha: Not all the means are equal where: 1 = mean number of washes using Type 1 wax 2 = mean number of washes using Type 2 wax 3 = mean number of washes using Type 3 wax

7. = (29 + 30.4 + 30)/3 = 29.8 Testing for the Equality of k Population Means: A Completely Randomized Experimental Design • Mean Square Between Treatments Because the sample sizes are all equal: {(29–29.8)2 + (30.4–29.8)2 + (30–29.8)2 }/2= .52 Mean Square Between Treatments • Mean Square Error of Within Treatments Within-treatments estimate of σ2 = {(2.5) + (3.3) + (2.5)}/3 = 33.2 MSE = 33.2/(15 - 3) = 2.77

8. Testing for the Equality of k Population Means: A Completely Randomized Experimental Design • Rejection Rule p-Value Approach: Reject H0 if p-value < .05 Critical Value Approach: Reject H0 if F> 3.89 where F.05 = 3.89 is based on an F distribution with 2 numerator degrees of freedom and 12 denominator degrees of freedom

9. Testing for the Equality of k Population Means: A Completely Randomized Experimental Design • Test Statistic F = MSTR/MSE = 2.60/2.77 = .939 • Conclusion The p-value is greater than .10, where F = 2.81. (Excel provides a p-value of .42.) Therefore, we cannot reject H0. There is insufficient evidence to conclude that the mean number of washes for the three wax types are not all the same.

10. Testing for the Equality of k Population Means: A Completely Randomized Experimental Design • ANOVA Table Source of Variation Sum of Squares Degrees of Freedom Mean Squares p-Value F 5.2 .42 Treatments 2 2.60 .939 Error 33.2 12 2.77 Total 38.4 14

11. Testing for the Equality of k Population Means: An Observational Study • Example: Reed Manufacturing Janet Reed would like to know if there is any significant difference in the mean number of hours worked per week for the department managers at her three manufacturing plants (in Buffalo, Pittsburgh, and Detroit). An F test will be conducted using a = .05.

12. Testing for the Equality of k Population Means: An Observational Study • Example: Reed Manufacturing A simple random sample of five managers from each of the three plants was taken and the number of hours worked by each manager in the previous week is shown on the next slide. Factor . . . Manufacturing plant Treatments . . . Buffalo, Pittsburgh, Detroit Experimental units . . . Managers Response variable . . . Number of hours worked

13. Testing for the Equality of k Population Means: An Observational Study Plant 3 Detroit Plant 2 Pittsburgh Plant 1 Buffalo Observation 1 2 3 4 5 48 54 57 54 62 51 63 61 54 56 73 63 66 64 74 Sample Mean 55 68 57 Sample Variance 26.0 26.5 24.5

14. Testing for the Equality of k Population Means: An Observational Study • p -Value and Critical Value Approaches 1. Develop the hypotheses. H0:  1= 2= 3 Ha: Not all the means are equal where:  1 = mean number of hours worked per week by the managers at Plant 1  2 = mean number of hours worked per week by the managers at Plant 2   3 = mean number of hours worked per week by the managers at Plant 3

15. = (55 + 68 + 57)/3 = 60 Testing for the Equality of k Population Means: An Observational Study • p -Value and Critical Value Approaches a = .05 2. Specify the level of significance. 3. Compute the value of the test statistic. Mean Square Due to Treatments (Sample sizes are all equal.) {(55 - 60)2 + (68 - 60)2 + (57 - 60)2 }/2= 49

16. Testing for the Equality of k Population Means: An Observational Study 3. Compute the value of the test statistic. (con’t.) Mean Square Due to Error MSE = {(26.0) + (26.5) + (24.5)}/3 = 25.667 F = MSTR/MSE = 245/25.667 = 9.55