Safe Lumber Company hires an independent contractor to monitor the groundwater within a one mile radius of its factory to ensure that no toxic levels of arsenic are being released. The contractor takes a sample of groundwater from selected sites within the one mile radius.
a) Suppose that data from 25 sites had arsenic levels averaging 75 ppb with a sample SD of 30 ppb. Construct a 95% confidence interval for the arsenic level.
b) Suppose that data from 9 sites had arsenic levels averaging 75 ppb with a sample SD of 30 ppb. Construct a 95% confidence interval for the arsenic level.