A couple of asides on notation and terminology. First, I have called the intercept α and the slopes βs to agree with the notation of the last chapter. But I have also included α in my numbering of the coefcients, as if it were β1. Indeed, there is nothing wrong with calling it β1, if that makes more sense to you. The important point is that there are k coefcients, of which k – 1 are slopes. You may well come across other authors who count things differently, α, or β0, might be the intercept and β1… βk the slopes. In that case, there would be k + 1 coefcients, of which k are slopes. All degrees of freedom would seem to be off by one. So, as you move beyond this book, always make sure you know whether k is the number of coefficients or the number of slopes. In this book, it is always the number of coefficients.