====卡方分布====
====Chi-square distribution====
====1.定义(Denfinition)====
  *由 自由度df决定的正偏态分布。

  *A positively skewed distribution determined by the degrees of freedom (df).

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====2.特点(Characteristic)====
  * **数值**：X²都是正值。
  * **可加性**：几个X²的变量的和仍然服从卡方分布。
  * **形状**：
  * ① 取决于类目数，而不是样本数。（df=C-1）
  * ② 卡方分布呈现正偏态，右端无限延伸，不与基线相交。
  * ③ df越大，分布越趋向对称，越趋向正态分布。
  * **临界值**：df增加，临界值增加

  * **Numerical value**: X² are positive.
  * **Additivity**：The sum of several X² variables still obeys a chi-square distribution.
  * **Shape**：
  * ① Depends on the number of classes, not the number of samples.（df=C-1）
  * ② The chi-square distribution exhibits a positive skewness with an infinite extension at the right end that does not intersect the baseline.
  * ③ The larger df is, the more the distribution tends to be symmetric and normal.
  * **Threshold value**：Increasing df results in an increase in the critical value.


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====3.例图（Example chart）====

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