In Chapters 3 and 4 we discussed procedures for measuring the central tendency and the variability of distributions. In this chapter we discuss ways of describing the degree of relationship, or association, between two variables. Consider, for instance, the data in Table 5 .l, which lists the performances of eighteen children on an initial test of academic ability and a subsequent test achievement. How can we describe the degree to which "ability" and "achievement" are related in this example?