Construct 90% and 95% confidence intervals for the population mean number of days the car model sits on the dealership’s lot in Example 3. Compare the widths of the confidence intervals.

a. Find tc and E for each level of confidence.

b. Use x̅ and E to find the left and right endpoints of each confidence interval.

c. Interpret the results and compare the widths of the confidence intervals.

Example 3

You randomly select 36 cars of the same model that were sold at a car dealership and determine the number of days each car sat on the dealership’s lot before it was sold. The sample mean is 9.75 days, with a sample standard deviation of 2.39 days. Construct a 99% confidence interval for the population mean number of days the car model sits on the dealership’s lot.