The degrees of freedom in a multiple regression equals $N-k-1$, where $k$ is the number of variables.
Does $k$ include the response variable (i.e., $Y$)? For example, in the model $Y = B_0 + B_1X_1 + B_2X_2$, then does $k = 3$ (i.e., 1 df each for $Y$, $X_1$, & $X_2$)?
Best Answer
It's the number of predictor (x) variables; the additional -1 in the formula is for the intercept - it's an additional predictor. The Y doesn't count. So in your example $k=2$ and the error df is $N-3$