Greg goldman, research analyst in the fixed-income area of an investment bank, needs to determine the average duration of a sample of twenty 15-year fixed-coupon investment grade bonds. goldman first categorizes the bonds by risk class and then randomly selects bonds from each class. after combining the bonds selected (bond ratings and other information taken as of march 31 of the current year), he calculates a sample mean duration of 10.5 years. assuming that the actual population mean duration is 9.7 years, which of the following statements about goldman's sampling process and sample is least accurate? a. goldman is using time-series data. b. the sample mean is a random variable. c. the sampling error is 0.8 years.