Lab 7:Confidence Intervals

All confidence intervals must be reported as a complete sentence. Points will be deducted if not.

Exercise 1

A company that produces white bread is concerned about the distribution of the amount of sodium in its bread.The company takes a simple random sample of 100 slices of bread and computes the sample mean to be 103 milligrams of sodium per slice. Construct a 99% confidence interval for the unknown mean sodium level assuming that the population standard deviation is 10 milligrams.

Exercise 2

Fill in the blanks with one of the following: increases,decreases, orstays the same. Hint -- consider the margin of error formula: .

a. As the sample size (n) increases, the margin of error (E) .

b. As the confidence level (C) increases, the margin of error (E) .

c. As the standard deviation () increases, the margin of error (E) _________.

Exercise 3

You work for a consumer advocate agency and want to find the mean repair cost of a washing machine. In the past, the standard deviation of the cost of repairs for washing machines has been $17.50. As part of your study, you randomly select 40 repair costs and find the mean to be $100.00. Calculate a 90% confidence interval for the population mean.

Exercise 4

The actual time it takes to cook a ten pound turkey is a normally distributed. Suppose that a random sample of 19 ten pound turkeys is taken. Given that an average of 2.9 hours and a standard deviation of .24 hours was found for a sample of 19 turkeys, calculate a 90% confidence interval for the average cooking time of a ten pound turkey.

Exercise 5

The football coach randomly selected eight players and timed how long it took to perform a certain drill. The times in minutes were:

10	6	8	7
6	5	7	8

Assuming that the times follow a normal distribution, find a 95% confidence interval for thepopulation mean. (use 1-Var Stats for Sx)

Exercise 6

Randomly selected students participated in an experiment to test their ability to determine when 1 minute has passed.Forty students yielded a sample mean of 58.3 seconds (assume). Based a 95% confidence interval, is it likely to have an overall mean of 1 minute?

Exercise 7

When 14 different second-year medical students measured the blood pressure of the same person, they obtained the results listed below.Assuming the true standard deviation is known to be 10 mmHg, construct a 90% confidence interval of the true mean blood pressure for the patient.

SAMPLE: 138, 130, 135, 140, 120, 125, 120, 130, 130, 144, 143, 140, 130, 150

Exercise 8

What sample size is needed to estimate the mean white blood cell count (in cells per microliter) for the population of adults in the United States? Assume you want a 99% confidence interval that the sample mean is within 2 cells/L of the true mean. The population standard deviation is given as 2.5 cells/L.