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？
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.
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......
发表于 2009-5-24 10:37
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