How to construct a confidence interval in excel

Statistics Excel Program

Instructions
Confidence Interval for the Population Mean
Project Description:
The present study shows data for delivery times of international shipments for a logistics company. The company promotes its international delivery as a service that takes up to 7 days. After receiving multiple complaints that the delivery time is exceeding 7 days, the quality assurance team decided to collect data from a random sample to analyze the delivery times and decide if they should adjust the delivery time the company promises customers with. We will find the average and standard deviation for the delivery times for the random sample of shipments. We will find values from the probability table using excel functions to construct confidence intervals using different confidence levels. We will use the confidence intervals to make a conclusion about the population. We will also analyze the width of the confidence levels. Assume that the distribution of the delivery times is normal and the sample is randomly selected.
Instructions:
Use a cell reference or a single formula where appropriate in order to receive full credit. Do not copy and paste values or type values, as you will not receive full credit for your answers. For the purpose of grading the project, you are required to perform the following tasks:
Step Instructions Points
Possible
1 Start Excel. Download and open the workbook named: 0
Confidence Interval for the Population Mean Start
2 In cell B3, find the average delivery time for the sample of shipments from the Data sheet. 5
3 In cell B4, find the standard deviation of the delivery times for the sample of shipments from the Data sheet. 5
4 In cell B5, find the size of the sample of shipments from the Data sheet. 5
Hint: use the COUNT function.
5 To build the confidence interval for the mean of the delivery times from the Data Sheet, what is the appropriate probability table to use? Choose the correct answer from the dropdown menu in cell B6. 6
6 In cell B7, find the score from the appropriate probability table to construct a 90% confidence interval. 5
Hint: use the T.INV.2T function.
7 In cell B8, find the margin of error at the 90% confidence level. 5
Hint: use the CONFIDENCE.T function.
8 In cell B9, find the upper limit for the mean at the 90% confidence level. 5
9 In cell B10, find the lower limit for the mean at the 90% confidence level. 5
10 Based on the number in cell B9 and B10, we can be 90% confident that: 5
Choose the correct answer from the dropdown menu in cell B11.
11 In cell B12, find the width of the confidence interval at the 90% confidence level. 5
12 In cell B13, find the score from the appropriate probability table to construct a 99% confidence interval. 5
13 In cell B14, find the margin of error at the 99% confidence level. 5
14 In cell B15, find the upper limit for the mean at the 99% confidence level. 5
15 In cell B16, find the lower limit for the mean at the 99% confidence level. 5
16 In cell B17, find the width of the confidence interval at the 99% confidence level. 5
17 By comparing the width of the confidence interval (CI) at 90% and 99%, what can we conclude about the width of the CI? Choose the correct answer from the dropdown menu in cell B18. 6
18 Based on the upper and lower limit values for a 90% confidence level, should the quality assurance team consider recommending that the company should change their promise to customers that they deliver international shipments within 7 days? Choose the correct answer from the dropdown menu in cell B19. 6
19 Based on the upper and lower limit values for a 99% confidence level, should the quality assurance team consider recommending that the company should change their promise to customers that they deliver international shipments within 7 days? Choose the correct answer from the dropdown menu in cell B20. 6
20 What options does the quality assurance team have to reduce their margin of error for the 99% confidence interval? Choose the correct answer from the dropdown menu in cell B21. 6
21 Save your file and submit for grading. 0
Total Score 100
Data
Shipment Time to Deliver (Days)
1 7.0
2 12.0
3 4.0
4 2.0
5 6.0
6 4.0
7 2.0
8 4.0
9 4.0
10 5.0
11 11.0
12 9.0
13 7.0
14 2.0
15 2.0
16 4.0
17 9.0
18 5.0
19 9.0
20 3.0
21 6.0
22 2.0
23 6.0
24 5.0
25 6.0
26 4.0
27 5.0
28 3.0
29 4.0
30 6.0
31 9.0
32 2.0
33 5.0
34 6.0
35 7.0
36 2.0
37 6.0
38 9.0
39 5.0
40 10.0
41 5.0
42 6.0
43 10.0
44 3.0
45 12.0
46 9.0
47 6.0
48 4.0
49 3.0
50 7.0
51 2.0
52 7.0
53 3.0
54 2.0
55 7.0
56 3.0
57 5.0
58 7.0
59 4.0
60 6.0
61 4.0
62 4.0
63 7.0
64 8.0
65 4.0
66 7.0
67 9.0
68 6.0
69 7.0
70 11.0
71 9.0
72 4.0
73 8.0
74 10.0
75 6.0
76 7.0
77 4.0
78 5.0
79 8.0
80 8.0
81 5.0
82 9.0
83 7.0
84 6.0
85 14.0
86 9.0
87 3.0
88 4.0
Questions
Confidence Interval for the Population Mean
Use a cell reference or a single formula where appropriate in order to receive full credit. Do not copy and paste values or type values, as you will not receive full credit for your answers.
1-2. In cell B3, find the average delivery time for the sample of shipments from the Data sheet.
3. In cell B4, find the standard deviation of the delivery times for the sample of shipments from the Data sheet.
4. In cell B5, find the size of the sample of shipments from the Data sheet.
5. To build the confidence interval for the mean of the delivery times from the Data Sheet, what is the appropriate probability table to use?
6. In cell B7, find the score from the appropriate probability table to construct a 90% confidence interval.
7. In cell B8, find the margin of error at the 90% confidence level.
8. In cell B9, find the upper limit for the mean at the 90% confidence level.
9. In cell B10, find the lower limit for the mean at the 90% confidence level.
10. Based on the numbers in cell B9 and B10, we can be 90% confident that
11. In cell B12, find the width of the confidence interval at the 90% confidence level.
12. In cell B13, find the score from the appropriate probability table to construct a 99% confidence interval.
13. In cell B14, find the margin of error at the 99% confidence level.
14. In cell B15, find the upper limit for the mean at the 99% confidence level.
15. In cell B16, find the lower limit for the mean at the 99% confidence level.
16. In cell B17, find the width of the confidence interval at the 99% confidence level.
17. By comparing the width of the confidence interval (CI) at 90% and 99%, what can we conclude about the width of the CI?
18. Based on the upper and lower limit values for a 90% confidence level, should the quality assurance team consider recommending that the company should change their promise to customers that they deliver international shipments within 7 days?
19. Based on the upper and lower limit values for a 99% confidence level, should the quality assurance team consider recommending that the company should change their promise to customers that they deliver international shipments within 7 days?
20. What options does the quality assurance team have to reduce their margin of error for the 99% confidence interval?
Sheet1
As the confidence level increases, the width of the CI increases.
As the confidence level increases, the width of the CI decreases.
The width of the CI does not depend on the confidence level.
Yes, because the interval does not contain 7 days.
Yes, because the interval contains 7 days.
No, because the upper limit of the CI is less than 7 days.
No, because the upper limit of the CI is greater than 7 days.