In the following pages you will be introduced to the theory behind the technique known as Analysis of Variance (ANOVA). In subsequent chapters, you will be shown how to compute the statistic for simple experiments. The arithmetic is straightforward but can be time consuming and so, for more complicated designs, you will be shown how to interpret analyses instead. Throughout the rest of the book, the word factor will be preferred to the phrase independent variable. The word level will be preferred to the word condition. Hence, a study with two independent variables, each with two conditions, would be described as having two factors, each with two levels. Since any experiment must have at least two conditions, it follows that a factor must always have at least two levels.