Equating adjusts for differences in difficulty, not differences in content. That statement on page 3 is one of the most important sentences in the introductory chapter of this book. To a large extent this chapter considers situations in which statistical adjustments are made to scores for tests that differ in content and/or difficulty, and usually both. In some cases, these differences are relatively small; in most cases, tests clearly measure very different content/constructs. We refer generically to a relationship between scores on such tests as a linking.