degrees of freedom

Denoted $df$

The number of scores in a sample that reflect the variability of the population. This is used to determine the shape of the Sampling Distributions for estimating ${\sigma }_x$ for t-tests.

e.g. $N-1$

$df=\frac{(\frac{(s_1{)}^2}{n_1}+\frac{(s_2{)}^2}{n_2}{)}^2}{(\frac{1}{(n_1-1)})(\frac{(s_1{)}^2}{n_1}{)}^2+(\frac{1}{n_2-1})(\frac{(s_2{)}^2}{n_2}{)}^2}$