You already know how to fit a straight line to a scatterplot and describe the relationship between two quantitative variables. But that line is just one view of the data. If we took a new sample, the line would change. So the big question is: does the relationship we see reflect something real, or could it just be randomness? In this chapter, we will build tools to test the slope, estimate average responses, predict new individual values, and check whether the model behind these tools actually holds up.