12# luckyli2005
You are using the excel functions in a wrong way. The function VARP and COVAR ignore the probability of the random variables. They simply use mean and quadratic error to do the estimation, which is not what you want. When you see variance of B is 0 (D12 in your excel file), you should know it is very wrong.
Google translator:
您使用的是Excel的一个错误的单向函数。函数VARP和COVAR忽略了随机变量的概率。他们只是用二次误差和均值做的估计,这不是你想要的。当你看到B的方差为0,你应该知道这是非常错误的。
萧条 | 0.15 | -0.2 |
景气 | 0.5 | 0.7 |
正常 | 0.35 | 0.4 |
Using A for example, the correct method should be
例如使用A,正确的方法应该是
E(A)=-0.2*0.15+0.7*0.5+0.4*0.35=0.46
E(A^2)=0.4*0.15+0.49*0.5+0.16*0.35=0.3610
Var(A)=E(A^2)-(E(A))^2=0.3610-0.46*0.46=0.1494