Square difference: a? -B? =(a+b)(a-b). Text expression: The product of the sum of two numbers and the difference of two numbers is equal to the square of the difference of two numbers. This is the square difference formula.
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.
Extended data:
Because the variance is the square of the data, it is generally too different from the detected value itself, so it is difficult for people to measure it intuitively, so it is often converted back by the root sign of the variance (taking the arithmetic square root). This is the standard deviation (SD) we are going to talk about.
In statistics, the average difference of samples is mostly divided by the degree of freedom (n- 1), which refers to the degree to which samples can be freely selected. When there is only one left, it can no longer be free, so the degree of freedom is (n- 1).
Baidu encyclopedia-standard deviation formula