Some accounting firms give the client an option to pay a fee when the tax return is completed that guarantees tax advices and support from the accountant if the client were audited. A large accounting firm is trying to determine what fee to charge for next year's returns. In previous years, the actual mean cost to the firm for attending a client audit session was $680. To determine if this cost has changed, the firm randomly samples 35 client audit fees. The sample mean audit cost was $710 with a standard deviation of 75
1) Develop a 90% confidence interval estimate for the mean audit cost. ($__,$__)
2) Based on your confidence interval, what do you think of the claim that the mean cost has changed?
- The interval does not contain the historical data mean $710, which supports claim the mean cost has changed.
- The interval contains historical data mean $710, which supports the claim the mean cost has changed.
- The interval does not contains historical data mean $710, which does not support the claim it has changed.
- The interval contains historical data mean $690, which does not support the claim the mean cost has changed.
1)
sample mean, xbar = 710
sample standard deviation, s = 75
sample size, n = 35
degrees of freedom, df = n - 1 = 34
Given CI level is 90%, hence α = 1 - 0.9 = 0.1
α/2 = 0.1/2 = 0.05, tc = t(α/2, df) = 1.691
ME = tc * s/sqrt(n)
ME = 1.691 * 75/sqrt(35)
ME = 21.4373
CI = (xbar - tc * s/sqrt(n) , xbar + tc * s/sqrt(n))
CI = (710 - 1.691 * 75/sqrt(35) , 710 + 1.691 * 75/sqrt(35))
CI = (688.56 , 731.44)
2)
- The interval contains historical data mean $690, which does not support the claim the mean cost has changed.
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