# Confidence Interval for z and t test

Run Excel and answer following questions. You need this problem sheet, answers, and the Excel output tables to take Webtest 3. You can use textbook, course slides as well.Webtest 3 will be open from Sat. 11/23 through Sun 11/24via Hawkes Learning System.

Most computer laptops use a rechargeable lithium-ion battery. However, with brighter computer displays and more powerful chips, batteries are not lasting any longer than they did on older computer laptop models.To determine the average number of hours of use that college student gets on a laptop before the batteries fail, 70students were randomly selected, and values of the variable LaptopBatteryLife were recorded.

Use the data file named Excel Assignment 3 dat..xls. The variable LaptopBatteryLife is defined as the number of hours that a laptop computer will function.

a. The population standard deviation is believed to be 0.8 hours. Now what is the 95% confidence interval for the mean lifetime? Interpret it.

What is the margin of error in this confidence interval? Calculate it using information from the output table

b. The population standard deviation is believed to be 0.8 hours. What is the 80% confidence interval for the mean lifetime? Interpret it.

What is the margin of error in this confidence interval? Calculate it using information from the output table

c. Compare confidence intervals from problem a and b. Which is wider?

d. Determine a 95% confidence interval for the mean battery lifetime assuming the population standard deviation is not known. Interpret it.

What is the margin of error in this confidence interval?

e. Determine a80% confidence interval for the mean battery lifetime assuming the population standard deviation is not known. Interpret it.

What is the margin of error in this confidence interval?

f. Compare confidence intervals from problem d and e. Which is wider?

Excel Instruction

You have done step 1 and 2 for Excel Assignment 1.

1. Download Excel Macro file and save it in your computer or memory stick.

2. Bring Add-ins and Macro in the Excel. (We did it with assignment 1).

(open Assignment 3 data file click File tab from the top menu select optionsif you forgot details, refer to PowerPoint instruction file...)

If you see Add-Ins tab at the top menu bar of Excel page, it means that step 1 and 2 were successfully done.

3. Download Assignment 3 data file and save it as your own.

4. Excel steps are described in Textbook chapter 8 Appendix (page 325-326) using different example. Or you can refer to lecture slide Excel Macro part in 8.3 and 8.4 sections of chapter 8.

• Confidence Intervals for the Mean with Known Population Standard Deviation

:click Add-Ins tabKPK Data AnalysisOne Population InferenceAt Test for inference on, choose population mean (z statistic); assign input range; check label ; put population standard deviation; put alpha level (you learned this concept in lecture); check confidence interval; assign output range. Click ok.

• Confidence Intervals for the Mean with UnknownPopulation Standard Deviation

: follow same process except selecting population mean withest.std.dev.( t statistic)at Test for inference on:.

#### Solution Preview

Run Excel and answer following questions. You need this problem sheet, answers, and the Excel output tables to take Webtest 3. You can use textbook, course slides as well. Webtest 3 will be open from Sat. 11/23 through Sun 11/24 via Hawkes Learning System.

Most computer laptops use a rechargeable lithium-ion battery. However, with brighter computer displays and more powerful chips, batteries are not lasting any longer than they did on older computer laptop models. To determine the average number of hours of use that college student gets on a laptop before the batteries fail, 70 students were randomly selected, and values of the variable LaptopBatteryLife were recorded.

Use the data file named Excel Assignment 3 dat..xls. The variable LaptopBatteryLife is defined as the number of hours that a laptop computer will function.

a. The population standard deviation is believed to be 0.8 hours. Now what is the 95% confidence interval for the mean lifetime? Interpret it.

What is the margin of error in this confidence interval? ...

#### Solution Summary

This solution is comprised of a detailed explanation of confidence interval using the KPK Data Analysis which is a excel add ins. This solution mainly discussed the 95% and 80% confidence interval using z and t test. A logical answer is given for every situation for better clarity on using the interpretation of confidence interval.