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?