TESTING HYPOTHESES

Sometimes you have information available for everyone or everything that you are interested in drawing conclusions about, and all you need to do is summarize your data. But usually that is not the case. Instead, you usually want to draw conclusions about much larger groups of people or objects than those included in your study. You want to know what proportion of all purchasers of your product are satisfied with it, based on the opinions of the relatively small number of purchasers included in your survey. You want to know whether buyers of your product differ from nonbuyers. Are they younger , richer, better educated ? You want to be able to draw conclusions about all buyers and nonbuyers based on the people you have included in your study.

To do this (and understand it), you have to learn something about statistical inference. Later chapters in this volume will show you how to test hypotheses and draw conclusions about populations based on samples. You will learn how to test whether you have sufficient evidence to believe that the differences or relationships you find in your sample are true for the whole population.