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Falcon · 2019年11月04日

问一道题:NO.PZ201709270100000305 第5小题 [ CFA II ]

* 问题详情,请 查看题干

问题如下图:

选项:

A.

B.

C.

解释:

只要是k增加,adjusted R2一定下降,这么理解对吗?

题目的相关系数表和自由度k有什么关系呢?

2 个答案
已采纳答案

星星_品职助教 · 2019年11月06日

同学你好,

回复下追问的问题哈。答案解析这个公式的主要用途是体现R2和adj R2的关系。K增加会导致n-k-1减少,这个影响的方向会使得adj R2减小。但K增加会使得R2上升,这个方向的变化会使得adj R2增加,所以主要要看新增变量对于模型的解释力度(R2的增加)是不是会大过n-k-1造成的影响。

另外,这个公式最多的应用其实是在计算上,是当题干已知R2时,可以迅速的得出adj R2。加油






星星_品职助教 · 2019年11月05日

同学你好,

如果K也就是X的个数增加,那么可以得出结论为R2一定会增加。但Adj R2不一定增加。

原因K增加对Adj R2有双重的影响。Adj R2的公式里K在分母上,相当于K增加会有一个使得Adj R2下降的效果。但如果增加的变量对于公式解释及其有帮助,那么也会使得regression的标准差减小。这个角度对Adj R2又有个提升的效果。

所以增加一个变量,对adj R2起的效果是双向的。结论为,如果新增加的变量对模型的边际贡献很大,那么adj R2会上升,如果新增加的变量对模型的解释力度贡献微乎其微,那么adj R2就会下降,这其实也是建立模型时一种排除非必要变量的方法。

对于本题来说,相关系数矩阵的用处是在解释新增加的变量Div跟Y的相关性很小,只有0.117,所以新增这个变量对Y的解释力度很小,这个时候就会导致Adj R2下降。加油

Falcon · 2019年11月06日

新增变量对Y能很好解释的话,能使adjusted R2增加。在这个公式里看不出来啊,是不是这个公式的缺陷

星星_品职助教 · 2019年11月06日

同学你好,字数比较多,新起了个回复哈~

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