There are two formulas for variance calculation: s 2 = (1/n) and S=(X2 average) 2. 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.

Probability theory is a branch of mathematics that studies the quantitative laws of random phenomena. Random phenomenon is relative to decisive phenomenon, and the inevitable occurrence of a certain result under certain conditions is called decisive phenomenon. For example, at standard atmospheric pressure, when pure water is heated to 100℃, water will inevitably boil. Random phenomenon means that under the same basic conditions, before each experiment or observation, it is uncertain what kind of results will appear, showing contingency.