If what follows reads like a mathematics text, well, to some extent it is. Our goal is to explain all (or at least most) of the mathematical principles used herein, rather than peppering the main text of the book with occasional lessons in math. If you want to understand the details of our calculations, feel free to read on. If not, you can just take our word that we're doing our calculations correctly. We emphasize that the math in this section is included for the sake of showing our work; if you wish to skip over some parts where the math gets too intense, feel free to do so.

Measuring Average, Standard Deviation, and Variance

We begin by defining some mathematical terms frequently seen throughout the book. We'll begin with the concept of “average” (which is the same as the “mean”). Most of you are probably familiar with this term. Given a series of values, their average equals their sum divided by the number of values in the series. For example, if you wish to average 0, 0, 2, 3, and 5, you divide their sum (0+0+2+3+5=10) by 5 to get 2. In mathematical terms, you calculate the average using the following expression: