## Graphs of Two Variables The Coordinate System

Although the three graphs in Figure A-l are useful in showing how a variable changes over time or across individuals, such graphs are limited in how much they can tell us. These graphs display information only on a single variable. Economists an- often concerned with the relationships between variable». Thus, they need to display two variables on a single graph. The coordinate system makes thb possible.

Suppose you want to examine the relationship between study time and grade point average. For each student in your class, you could record a pair of numbers: hours per week spent studying and grade point average. These numbers could then be placed in parentheses as an ordered/wand appearasa single point on the graph. Albert E., for instance, is represented by the ordered pair (25 hours/week. 3.5 CPA), while his "what-me-worry?" classmate Alfred E. is represented by the ordered pair {5 hours/week. 2X>CPA).

We can graph these ordered pairs on a two-dimensional grid. The first number in each ordered pair, called the xtvorJinji/e. tells us the horizontal location of the point. The second number, called the y-cwrdinafe, tells us the vertical location of the point. Ihe point with both an *-coordinate and a y-coordinate of zero is known as the origin. The two coordinates in the ordered pair tell us where the point is located in relation to the origin: * units to the right of the origin and y units above it.

Figure A-2 graphs grade point average against study time for Albert L'., Alfred E., and their classmates. This type of graph is called a saitterpiot because it plots scattered points. Looking at this graph, we immediately notice that points farther to the right (indicating more study time) also tend to be higher (indicating a better

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