But why even bother creating a formula (a line here) to approximate data that does not satisfy that formula? Remember that you would expect collected data to vary slightly as one repeatedly collects that data in the same way that you would expect to make a slightly different score on repeated attempts at exams on the same material. Creating a forumla that is close to your data gives a well-defined way to predict a y value for a given x value. This predictive behavior is illustrated in the exercise below.