CASE STUDY Are people with larger brains more intelligent? People have investigated this question throughout history. To answer it, we must measure “intelligence.” This requires us to reduce the vague idea to a number that can go up or down. The first step is to say what we mean by intelligence. Does a vast knowledge of many subjects constitute intelligence? How about the ability to solve difficult puzzles or do complicated mathematical calculations? Or is it some combination of all of these?

Once we decide what intelligence is, we must actually produce the numbers. Should we use the score on a written test? Perhaps a formula that also includes grades in school would be better. Not only is it hard to say exactly what “intelligence” is, but it’s hard to attach a number to measure whatever we say it is. And in the end, can we even trust the number we produce?

By the end of this chapter, you will have learned principles that will help you understand the process of measurement and determine whether you can trust the resulting numbers.