Our interest in this chapter is in situations in which we can associate to each element of a population or sample two measurements $$x$$ and y, particularly in the case that it is of interest to use the value of $$x$$ to predict the value of y. For example, the population could be the air in automobile garages, $$x$$ could be the electrical current produced by an electrochemical reaction taking place in a carbon monoxide meter, and $$y$$ the concentration of carbon monoxide in the air. In this chapter we will learn statistical methods for analyzing the relationship between variables $$x$$ and $$y$$ in this context.