Whirltag claims that the useful lifetime of its dishwashers is normally distributed with a mean of 3 years and a standard deviation 1 year. Consumer Rejects magazine tested 25 of these dishwashers and found that the average useful life was only 2.5 years. Using Whirltag's reported 1-year standard deviation, what is Consumer Reject's 95 percent confidence interval for the average lifetime of Whirltag dishwashers?
The standard deviation of a sample population is the standard deviation of the entire population divided by the square root of the number of samples. So
1/sqrt 25 = 0.2
In addition, we need ...
Statistical stimation of lifetime of a consumer product