A dataset with two variables contains what is
called bivariate data. This chapter discusses ways to describe
the relationship between two variables. For example, you may
wish to describe the relationship between the heights and weights
of people to determine the extent to which taller people weigh
more.

The introductory section gives more examples
of bivariate relationships and presents the most common way of
portraying these relationships graphically. The next five sections
discuss Pearson's correlation, the most common index of the relationship
between two variables. The final section, "Variance
Sum Law II,"
makes use of Pearson's correlation to generalize this law to
bivariate data.