Suppose demand increases but supply decreases, as shown to the right. Both shifts increase price, so the equilibrium price increases unambiguously. But a demand increase leads to an increase in the equilibrium quantity, while a supply decrease leads to a decrease; making the change in the equilibrium quantity indeterminate.

    In the graph the equilibrium quantity remains about the same. However, if demand had shifted out slightly more, the equilibrium quantity would have risen. If supply had shifted out slightly more, the equilibrium quantity would have fallen. Hence, the indeterminacy.

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