Df stands for degree of freedom, which is the abbreviation of English degree of freedom. It is the number of variables whose values are not limited when calculating the unified measure. The formula is df = n-K. Where n is the number of samples, and k is the number of constraints or variables, or the number of other independent statistical data used in calculating the unified measurement value. Used for sampling distribution.

Statistically, the degree of freedom refers to the number of independent or freely changeable data in the sample when the parameters of the population are estimated by the statistics of the sample, which is called the degree of freedom of statistics. Generally speaking, the degree of freedom is equal to the number of independent variables minus their derivatives.

Mathematically, the degree of freedom is the dimension of a random vector, that is, the minimum number of unit vectors required for a vector to be completely described.