3.众数：一组数中的众数是指出现频率最高的数。

　　例：the mode of 7，9，6，7，2，1 is 7。

　　4.值域：表明数的分布的量，其被定义为最大值减最小值的差。

　　例：the range of1，7，27，27，36 is 36-= 37。

　　5.标准方差：

　　One of the most common measures of dispersion is the standard deviation. Generally speaking, the greater the data are spread away from the mean, the greater the standard deviation. The standard deviation of n numbers can be calculated as follows:

　　find the arithmetic mean ;

　　find the differences between the mean and each of the n numbers ;

　　square each of the differences ;

　　find the average of the squared differences ;

　　take the nonnegative square root of this average.

　　Notice that the standard deviation depends on every data value, although it depends most on values that are farthest from the mean. This is why a distribution with data grouped closely around the mean will have a smaller standard deviation than data spread far from the mean.

　　6.排列与组合

　　There are some useful methods for counting objects and sets of objects without actually listing the elements to be counted. The following principle of Multiplication is fundamental to these methods.

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