The Case of Buying Computers
The purchasing manager of a company had been tasked with buying new computers for all of the employees. Needing to purchase about 500 computers, the manager sent out requests for proposals to three companies: Swell, Doorway and MBI. Each of these companies responded and the manager quickly saw that he could get the best deal from Doorway. Given that, he completed a purchase order and presented it to the Vice-President of Finance for her approval. Despite the good deal he had managed to work out, he was shocked to hear that the Vice-President wanted to go with MBI. "After all", the VP said, "MBI is the industry leader and we've always had fewer problems with their equipment than we've had with the computers we've bought from Swell and Doorway. Because of that, this company is going to guy MBI.".
The purchasing manager, really not believing what he had just heard, went to the director of technical support to look at the maintenance requests for the past five years. He was able to put together a chart showing the actual occurrence of maintenance requests for each of the brands of computer. These figures are shown in the chart below. The manager knew that the VP had final authority on the request but wanted to know, for his own peace of mind, if there were really any differences between the amount of maintenance each brand required.
Total
Maintenance
Requests
Doorway 33
1.What is the hypothesis that the manager is investigating?
2.What is the independent variable? What are the levels of the independent variable?
3.What is the dependent variable?
4.Which statistical test would he use to test his hypothesis?
5.For each of the sets of output below, what can you tell about the dependent variable? What decision would the manager make?
Case A:
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Observed Number
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Expected Number
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Residual
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Doorway
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33
|
25.7
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7.3
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Swell
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25
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25.7
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-.7
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MBI
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19
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25.7
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-6.7
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Total
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77
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Brand
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Chi-Square
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3.844
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Degrees of Freedom
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2
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p value
|
.146
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Case B:
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Observed Number
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Expected Number
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Residual
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Doorway
|
43
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25.7
|
17.3
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Swell
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21
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25.7
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-4.7
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MBI
|
13
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25.7
|
-12.7
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Total
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77
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Brand
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Chi-Square
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18.805
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Degrees of Freedom
|
2
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p value
|
.000
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Case C:
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Observed Number
|
Expected Number
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Residual
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Doorway
|
15
|
25.7
|
-10.7
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Swell
|
21
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25.7
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-4.7
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MBI
|
41
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25.7
|
15.3
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Total
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77
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Brand
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Chi-Square
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14.442
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Degrees of Freedom
|
2
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p value
|
.001
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