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[CFA入门] CFA L1 Mock Exam一道题请教

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!!!!

 because this is a sample, not population, for all sample questions, except for the calculation of means, remember to subtract 1 from n to calculate variance, covariance, etc.

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agree with LS,  and just want to mention it :    press 2ND 7( data )insert data  and press 2ND 8 (stat),  there are some figures for you directly, such sample variance for each group......

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太感谢了!!!

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怎么我用计算器一步一步摁出来的数,cov= r*std(x)*std(y), r=0.1395, std(x)=6.9776, std(y)=10.654,出来的结果是10.37 。。。。是A啊

 

哪里出错了吗

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