Question

A light bulb manufacturer wants to compare the mean lifetimes of two of its light bulbs, model A and model B. Independent random samples of the two models were taken. Analysis of 11 bulbs of model A showed a mean lifetime of 1361hours and a standard deviation of 83 hours. Analysis of 15 bulbs of model B showed a mean lifetime of 1304 hours and a standard deviation of 81hours. Assume that the populations of lifetimes for each model are normally distributedand that the variances of these populations are equal. Construct a 99% confidence interval for the difference −μ1μ2 between the mean lifetime μ1 of model A bulbs and the mean lifetime μ2 of model B bulbs. Then complete the table below. Carry your intermediate computations to at least three decimal places. Round your responses to at least two decimal places. (If necessary, consult a  list of formulas.)

What is the lower limit of the 99% confidence interval?
What is the upper limit of the 99% confidence interval?

Answer 1