themselves as an exact straight line, it often turns out that a straight line can offer a reasonable approximation

of actual data.

Interpreting the Slope

The concept of slope is very useful in economics, because it measures the relationship between two variables.

A positive slope means that two variables are positively related; that is, when x increases, so does y, or when x

decreases, y decreases also. Graphically, a positive slope means that as a line on the line graph moves from left

to right, the line rises. The length-weight relationship, shown in Figure A3 later in this Appendix, has a positive

slope. We will learn in other chapters that price and quantity supplied have a positive relationship; that is,
firms will supply more when the price is higher.

A negative slope means that two variables are negatively related; that is, when x increases, y decreases, or

when x decreases, y increases. Graphically, a negative slope means that, as the line on the line graph moves
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