Construct a 94% confidence interval for the difference between the mean lifetimes of two kinds of light bulbs, given that a random sample of 40 light bulbs of the first kind lasted on the average 418 hours of continuous use and 50 light bulbs of the second kind lasted on the average 402 hours of continuous use. The population standard deviations are known to be oj = 26 and oz = 22. Solution For a = 0.06, we find from Table III that zo.03 = 1.88. Therefore, the 94% confidence interval for ujhs #2 is 262 222 (418 - 402) - 1.88 . + 40 50