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Simultaneity and Identification: Creatine

In his latest weekly column (Nov 13, 2007, mostly about sports but also with a good dose of economics) at ESPN, Gregg Easterbrook writes,

Economics note: As the sense that creatine possesses forbidden powers has declined, so has its price. Five years ago, General Nutrition Centers sold a canister for $100 or more; now, canisters sell for $25 or so.

It looks as if Easterbrook is attributing the drop in the price of creatine to a decline in demand. But it is also quite likely that the high prices induced entry and increased competition among suppliers. Indeed, most websites devoted to body-building point out that there are now many brands and varieties of creatine to choose from.

If the demand curve shifted leftward, it also seems that the supply curve could have shifted to the right. Either shift (or both) would cause the price to drop, but it would be incorrect to attribute the price decline to either shift alone without more information.