[CFA入门] CFA L1 Mock Exam一道题请教 between, monthly, average, 题

jap0928

An analyst has gathered monthly returns for two stock indexes A and B:
Month
Returns for
Index A
Returns for
Index B
1 -6.4% -6.2%
2 6.6% 19.0%
3 12.9% -7.7%
4 3.2% 4.0%
The covariance between Index A and Index B is closest to:
A. 10.37.
B. 13.82.
C. 19.64.

Calculation of the covariance proceeds as follows:
1) Compute the average for each index:
Index A = (-6.4 + 6.6 + 12.9+3.2)/4 = 4.08
Index B = (-6.2 + 19.0 - 7.7 + 4)/4 = 2.28
2) Compute the following sum:
(-6.4-4.08) × (-6.2-2.28) + (6.6-4.08) × (19.0-2.28) + (12.9-4.08) × (-7.7-2.28) +
(3.2-4.08) × (4.0-2.28) = 41.47
3) Divide the sum found in 2) by number of observation minus one = 41.47/(4-1)
=13.82


不明白的是，最后一步，为什么要divide by "number of observation - 1" ??????
我翻遍了所有的复习资料好像都没有找到为什么啊？

我计算的时候算到了41.47那布， 但是看到答案中没有这个选项，所以就估计要除以4，结果还是错了....要除以3.....不明白为什么要除呢？请大牛们指教阿，非常感谢！！！！

jap0928

太感谢了！！！