apache-commons DescriptiveStatistics 给出了错误的 StandardDeviation？

Question

我有这个代码：

    DescriptiveStatistics stats = new DescriptiveStatistics( new double[] {2,4,4,4,5,5,7,9} );
    System.out.println("var="+stats.getVariance());
    System.out.println("sd="+stats.getStandardDeviation());

我从维基百科 https://en.wikipedia.org/wiki/Standard_deviation 中获取了示例。答案应该是var=4.0, sd=2.0，但是我得到的是：

    4.571428571428571
    2.138089935299395

我错过了什么？

Answer 1

getVariance()除以n-1，这里来自docs：

Returns the (sample) variance of the available values.

This method returns the bias-corrected sample variance (using n - 1 in the denominator). Use getPopulationVariance() for the non-bias-corrected population variance.

背景：通常，一个人有一个来自总体的样本，并且通过计算偏差校正（或无偏差）的样本方差，计算出的估计值的期望值等于总体方差。我为此 answer. And Wikipedia has background on population variance vs. sample variance.

编写了示例代码来演示这一点

apache-commons DescriptiveStatistics 给出了错误的 StandardDeviation？

apache-commons DescriptiveStatistics gives wrong StandardDeviation?

java

statistics

apache-commons