The expression to the left is a general expression for the own price elasticity of demand. In this case QD represents desired quantity demanded and P represents market price.

   Remember, the law of demand states that market price and desired quantity demanded are inversely related. Therefore, a positive (+) percentage change in market price will always be met by a negative (-) percentage change in desired quantity demanded, and vice versa. Thus, the signs of the numerator and denominator in the above expression will always be opposite; meaning the own price elasticity of demand must be negative, reflecting the sign of the slope of the demand curve.

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