Announcements for 84 upcoming engineering conferences were randomly picked from a stack of IEEE Spectrum magazines. The mean length of the conferences was 3.94 days, with a standard deviation of 1.28 days. Assume the underlying population is normal. What is the error bound of 95% confidence interval for the population mean length of engineering conferences? Construct a 95% confidence interval for the population mean length of engineering conferences. What is the lower bound? Construct a 95% confidence interval for the population mean length of engineering conferences. What is the upper bound?