Thus far, all the elasticities we've studied have been demand elasticities. It's also useful to know how strongly the desired quantity supplied responds to changes in market price.

   We know from the Law of Supply that desired quantity supplied and market price move in the same direction. Increases in price lead to increases in quantity supplied, and vice versa. This means that the own price elasticity of supply, , must always be positive.

   The expression to the left is the general version of supply elasticity measure. By now, you should see how to create any elasticity measure you wish. In this case, supply elasticity is the ratio of percentage change in desired quantity supplied to the percentage change in market price, and is always positive.

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