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[求助] 2级 有关Adjusted Beta and Mean Reverting Level ( Portfolio 的问题)

各位高人:先感谢看帖。

有个地方很疑惑。。。忘各位指点,感激涕零。 2级Notes Portfolio Management Book 3 上,Page 229, 有关Adjusted Beta的解释,各位请注意本页最下面的话, The mean-reverting level of the beta is 1. If the historical beta is greater than 1, then the adjusted Beta will be less than historical beta and closer to1. Vice versa..翻页后面...The adjusted Beta forecast will move forward 1 more quickly for larger values of alfa 0 (eg, as alfa 0 appraoches a value of 1) . 请大家注意红字的内容,这段话后面 ,还举了一个例子,用1/3,和2/3来证明Adjusted Beta 的确是比原来的Beta更接近1(也就是 Mean reverting level)了。 但是不知大家是否认真想过上面红字内容,真的是alfa 0 越大,那么 Adjusted Beta就能越接近1么?
1. 08年 practice exam Vol 1 Exam 2 Morning 18题目 Page 84,请看对于Estimated Alfa 0 的判断, 这道题基本对上面的红字采取了认同的态度。“ 题目中, 原来公司的alpha 0 是1/3,而另一个分析师的Alpha 0 是1/4, 那么这样就可以算出相应的Alpha 1,  两个式子分别是: 原公司Adjusted Beta = 1/3+2/3Beta, 分析师Adjusted Beta =
1/4+3/4Beta, 那么根据这个式子,答案就说分析师的能更快的接近1。 请看原题解答:  Using adjusted standard errors will change the t-statistic and potentially the statistical significance, but not the beta estimate itself. The mean-reverting level of the beta is 1. If the historical beta is greater than 1, then the
adjusted beta will be less than the historical beta and closer to 1. If the historical beta is less than 1, then the adjusted beta will be greater than the historical beta and closer to 1. The adjusted beta forecast will move toward 1 more quickly for larger values of α1. Since α0 = 1 – α1, then De Jong’s smaller value of α0 will result
in a larger value of α1, and her adjusted beta forecast will be closer to 1 than if she used the firm’s α0.
2. 但是事实果真如此么?请大家仔细看一下这两个简单的线性方程。。。。我用Excel做了个简单的示意图(如果论坛的帖子发成功我的图的话),。。。我要表达的意思就是,。。在Beta小于1的时候,,,事实上,
1/3+2/3Beta 这个方程更接近1....如果大家看图还是不够过瘾,可以代数0.9实验一下,,,, 1/3+2/3*1=0.93333 , 而1/4+3/4*0.9=0.925, 也就是说0.9333>0.925,更接近1.

请问。。。。。。CFA教材有误? 还是我疏忽什么了???? 多谢高人。多谢高人













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[此贴子已经被作者于2009-2-18 13:30:12编辑过]

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