Answer: With (a) confidence, the (b) (c) width of troughs is between (d) and (e) inches. Q2 A survey is planned to determine the mean annual family medical expenses of employees of a large company. The management of the company wishes to be 95% confident that the sample mean is correct to within ± $50 of the population mean annual family medical expenses. A previous study indicates that the standard deviation is approximately $400. a) How large a sample size is necessary? b) If management wants to be correct to within ± $25, what sample size is necessary? Q3 If a quality control manager wants to estimate the mean life of light bulbs to within ± 20 hours with 95% confidence and also assumes that the population standard deviation is 100 hours, what sample size is needed?