Suppose we want to fit the model y = ax + b to the data shown in Figure 3.4. How might we choose the constants a and b to determine the line that best fits the data? Generally, When more than two data point exist, not all of them can be expected to lie exactly along a single straight line, even if such a line accurately models the relationship between the two variables X and Y. Ordinarily, there will be some vertical discrepancy between a few of the data points and any particular line under consideration.