Читать книгу Biostatistics Decoded - A. Gouveia Oliveira - Страница 17

Degrees of freedom is a central notion in statistics that applies to all problems of estimation of quantities in populations from the observations made on samples. The number of degrees of freedom is the number of values in the calculation of a quantity that are free to vary. The general rule for finding the number of degrees of freedom for any statistic that estimates a quantity in the population is to count the number of independent values used in the calculation minus the number of population quantities that were replaced by sample quantities during the calculation.

In the calculation of the variance, instead of summing the squared differences of each value to the population mean, we summed the squared differences to the sample mean. Therefore, we replaced a population parameter by a sample parameter and, because of that, we lose one degree of freedom. Therefore, the number of degrees of freedom of a sample variance is n − 1.