习题：
　　Exercise:
　　Adam Farman has been asked to estimate the volatility of a technology stock index. He has identified a statistic which has an expected value equal to the population volatility and has determined that increasing his sample size will decrease the sampling error for this statistic. His statistic can best be described as:
　　A.       Unbiased and efficient.
　　B.       Unbiased and consistent.
　　C.      Efficient and consistent.
　　D.      Unbiased only.
　　解析：
　　Answer: B
　　Explanation: An unbiased estimator has an expected value equal to the true value of the population parameter. A consistent estimator is more accurate the greater the sample size. An efficient estimator has a variance of sampling distribution that is less than that of any other unbiased estimator.
　　知识点：
　　Desirable Properties of an Estimator
　　An unbiased estimator is one for which the expected value of the estimator is equal to the parameter you are trying to estimate.
　　An unbiased estimator is also efficient if the variance of its sampling distribution is smaller than all the other unbiased estimators of the parameter you are trying to estimate.
　　A consistent estimator is one for which the accuracy of the parameter estimate increases as the sample size increases.
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