Two variates may be related to each other, often one causing the other. The question about how much the X variate changes (increases or decreases) for a one-unit change in Y is analyzed by regression analysis. This chapter deals with how to determine the slope of a straight line that fits the XY points, how to determine an estimate of the variability of those points, and how to handle outliers. It shows how to compare the slopes of two or more lines. Finally, the important issue of when and when not to use ratios is discussed.