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習(xí)題：
Exercise:
An analyst collects a sample of 50 P/E ratios of stocks that are representative of the market. The mean P/E of these stocks is 20 and the standard deviation is 8.5. What is the 95 percent confidence interval for the mean P/E of stocks in this market?
A 17.21 to 22.79
B 17.64 to 22.36
C 18.02 to 21.98
D 18.31 to 21.69
解析：
Answer: B
Explanation:
知識(shí)點(diǎn)：
Confidence Interval
While it is impossible to learn the exact value of the population mean using only the information in a sample, one can use data from a random sample to construct a set of values that contains the true population mean with a certain pre-specified probability. Such a set is called a confidence set, and the pre-specified probability that the true population mean is contained in this set is called confidence level.
The confidence set turns out to be all the possible values of the mean between a lower and an upper limit, so that the confidence set is an interval, called a confidence interval.
If the population has a normal distribution with a known variance, a confidence interval for the population mean is:
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