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

耶鲁公开课 - 金融市场课程节选

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

耶鲁公开课 - 金融市场课程节选