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这是本文档旧的修订版！

对称分布（symmetrical distribution）

定义：在曲线中间画一条垂直线，使得该线两侧的曲线互为镜像的分布
Definition: Draw a vertical line in the middle of the curve so that the curves on both sides of the line are mirrored to each other.

偏态分布（skewed distribution）

定义：数据堆积在分布的一端，而另一端成为比较尖细的尾端
Definition: Data is piled up at one end of the distribution, and the other end becomes the thinner tail.
正偏态（positively skewed）：如左图所示，数据堆积在左侧，尾部在右的偏态分布。其尾端指向X轴的正数一端，故称正偏态分布。具有均值大于中位数的特点。
As shown in the left figure, the data is stacked on the left and tailed on the right. Its tail is pointed to the positive end of the X-axis, and here came the name positive skewed distribution. It has a higher mean than median.
负偏态（negatively skewed）：反之，如右图所示，如果数据堆积在右侧，尾部位于左边，则该分布被称为负偏态分布。具有均值小于中位数的特点。
As shown in the figure on the right, if the data is stacked on the right and the tail is on the left, the distribution is so called negatively skewed. It has a lower mean than median.
具体参考第三章：集中量数与差异量数
For more information,please check for the part of numbers of concentrations.

双峰分布（bimodal distribution）

定义：分布中的两个分数附近集中着较多的次数，以致次数分布曲线有两个隆起的峰
Definition:There are more times concentrated around the two fractions in the distribution, so that the number distribution curve has two raised peaks.

矩形分布（rectangle distribution）