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Plotting a Linear Relationship Between Two Variables Ä Ä To plot a linear relationship between two variables, use the algebraic formula for a straight line in a plane: y = a + b (x). The formula for the slope of a line is the change in the y-axis variable divided by the change in the x-axis variable, called rise over run. In economics, you can plot a simple demand curve—the relationship between the price of a good and the quantity a consumer wants to buy at each price. In this example, the demand curve can be plotted from a table showing how many hamburgers Bob would buy each week at various prices. The relationship described is a linear relationship between two variables because the plotted line is straight. From algebra, recall that the formula for a straight line is: y = a + b (x), where y is the price, a is the vertical intercept, b is the slope of the line, and x is the quantity. This graph shows a behavioral relationship because it describes Bob’s consumption behavior. The intercept of the line on the y-axis is the price at which Bob buys zero hamburgers. The slope describes Bob’s behavior when the price changes. It means that Bob increases his consumption by one hamburger for every $.50 decrease in price. You determine the slope by dividing the change in the y variable by the change in the x variable, or rise/run. www.compasslearning.com Copyright ã 2006, Thinkwell Corp. All Rights Reserved. 1157.doc –rev 11/07/2006