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understanding confidence intervals

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A company wants to determine a confidence interval for the average CPU time of its teleprocessing transactions. A sample of 70 random transactions in milliseconds is given below. Assume that the transaction times follow a normal distribution with a standard deviation of 600 milliseconds. Use Excel to determine a 98% confidence interval for the average CPU time in milliseconds. Round your answers to the nearest integer and use ascending order.

Qn2. A tax assessor wants to assess the mean property tax bill for all homeowners in a certain state. From a survey ten years ago, a sample of 28 property tax bills is given below. Assume the property tax bills are approximately normally distributed. Use Excel to construct a 95% confidence interval for the population mean property tax bill. Round your answers to two decimal places and use increasing order.

Qn3. In a survey of 603 adults, 98 said that they regularly lie to people conducting surveys. Create a 99% confidence interval for the proportion of adults who regularly lie to people conducting surveys. Use Excel to create the confidence interval, rounding to four decimal places.

Qn4. Suppose the weights of tight ends in a football league are normally distributed such that σ2=1,369. A sample of 49 tight ends was randomly selected, and the weights are given below. Use Excel to calculate the 95% confidence interval for the mean weight of all tight ends in this league. Round your answers to two decimal places and use ascending order.

i sixsigma

i sixsigma Added an answer on Jun 10,2022

Understanding confidence intervals

Answer 1:

Solution answers

Use Excel to calculate the 95% confidence interval, where α=0.05 and n=28.

1. Open Excel and enter the given data in column A. Find the sample mean, x¯, using the AVERAGE function and the sample standard deviation, s, using the STDEV.S function. Thus, the sample mean, rounded to two decimal places, is 1390.75 and the sample standard deviation, rounded to two decimal places, is 528.27.

2. Click on any empty cell, enter =CONFIDENCE.T(0.05,528.27,28), and press ENTER.

3. The margin of error, rounded to two decimal places, is 204.84. The confidence interval for the population mean has a lower limit of 1390.75−204.84=1185.91 and an upper limit of 1390.75+204.84=1595.59.

Thus, the 95% confidence interval for the population mean property tax bill is (1185.91, 1595.59).

Solutions: -

The confidence interval for the unknown population proportion p is (p^−z⋆p^(1−p^)n−−−−−−−−√,p^−z⋆p^(1−p^)n−−−−−−−−√). The confidence interval can be calculated using Excel.

1. Identify α. Click on cell A1 and enter =1−0.99 and press ENTER.

2. Thus, α=0.01. Enter the number of successes, x=98, and sample size, n=603, in the Excel sheet in cells A2 and A3. To find the proportion of successes, p^, click on cell A4 and enter =A2/A3 and press ENTER.

3. Thus, p^≈0.1625. Use the NORM.S.INV function in Excel to find z⋆. Click on cell A5, enter =NORM.S.INV(1−A1/2), and press ENTER.

4. The answer for z⋆, rounded to two decimal places, is z⋆≈2.58. To calculate the standard error, p^(1−p^)n−−−−−−−−√, click on cell A6 and enter =SQRT(A4∗(1−A4)/A3) and press ENTER.

5. The answer for the standard error, rounded to four decimal places, is p^(1−p^)n−−−−−−−−√≈0.0150. To calculate the margin of error, z⋆p^(1−p^)n−−−−−−−−√, click on cell A7 and enter =A5*A6 and press ENTER.

6. The answer for the margin of error, rounded to four decimal places, is z⋆p^(1−p^)n−−−−−−−−√≈0.0387. The confidence interval for the population proportion has a lower limit of A4−A7=0.1238 and an upper limit of A4+A7=0.2012. Thus, the 99% confidence interval for the population proportion of adults who regularly lie to people conducting surveys, based on this sample, is approximately (0.1238, 0.2012).

Confidence interval estimation.

A 95% confidence interval for μ is (x¯−zα/2σn−−√,x¯+zα/2σn−−√). Here, α=0.05, σ=37, and n=49. Use Excel to calculate the 95% confidence interval.

1. Open Excel, enter the given data in column A, and find the sample mean, x¯, using the AVERAGE function. Thus, the sample mean, rounded to two decimal places, is x¯=251.78.

2. Click on any empty cell, enter =CONFIDENCE.NORM(0.05,37,49), and press ENTER.

3. The margin of error, rounded to two decimal places, is zα/2σn−−√≈10.36. The confidence interval for the population mean has a lower limit of 251.78−10.36=241.42 and an upper limit of 251.78+10.36=262.14.

Thus, the 95% confidence interval for μ is (241.42, 262.14).

understanding confidence intervals

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