As I pointed out in Chapter 8, there are two main areas of statistics: (1) measuring things, especially measuring relationships between variables; and (2) estimating how likely it is that the measure you have gotten could have occurred by chance. In Chapters 8 and 9, we looked at the first of these aspects, and now we examine the second. (I must note parenthetically that, as in preceding chapters, I do not intend to teach you how to use statistical tests. Instead, I present the general logic of such tests to help you understand what they mean when you see them. This approach might also serve as a useful supplement to a statistics course, which sometimes teaches students how to use the tests without putting the tests in a broader perspective.)