The formula for finding d(x): D(x) = e (x 2)-(ex) 2, and d (x) is the variance. Variance is a measure of dispersion when probability theory and statistical variance measure random variables or a set of data. Variance in probability theory is used to measure the deviation between random variables and their mathematical expectations (that is, the mean value). The variance (sample variance) in statistics is the average value of the square of the difference between each sample value and the average value of all sample values.

Average, a statistical term, is a quantity that represents the trend of a group of data. Refers to the sum of all the data in a set of data divided by the number of this set of data. It is an indicator reflecting the trend of data concentration. The key to solve the problem of average application is to determine the "total amount" and the total number of copies corresponding to the total amount.