In the last three chapters, we have developed the ideas of inferencecondence intervals and tests of hypotheses with known probabilities of being right or wrong-by exploring what we can say about proportions and means. These intervals and tests involve a single sample proportion or mean, and so are traditionally called “one-sample” intervals and tests. In this chapter, we explore tests about differences in proportions and means between two groups. Because we are comparing two groups, these are traditionally called “two-sample” tests.