Elasticity.

Elasticities are simple measures that indicate the change in one quantity as a result of a change in another, ceteris paribus. Early work on demand measurement and price elasticity looked at agricultural products. These studies were useful because of the large price variations observed, which were caused by fluctuating crop yields combined with competitive market conditions, and which troubled farmers and the general population alike. They were made possible precisely because of these reasons; that is, because of widely fluctuating prices and quantities. [ABSTRACT FROM AUTHOR]