Suppose someone has been investing money for n years and xi is the return on the investment in a given year.
假设有人投资了n年,Xi是第i年的收益率
If you have a sample with n observations, it's the summation I = 1 to n of xi/n--that's the average.
如果你有n个观测值,对Xi从i=1到n求和再除以n
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