One type of study is concerned with the relation between price and consumers' demand, 503. — Illustrated in the study of potatoes, 505. — Price data should relate to a specific market, 508. — Allowance must be made for effects of other forces, 509. — Demand must be expressed in terms of rates and must cover all uses, 512. — Elasticity depends on market point at which it is measured, 512. — A second and more common type of study is concerned with “ speculators'” demand, 518. — Cannot be expressed in terms of rates, 519. — But may yield some knowledge of consumers' demand, 521. — Current “demand and supply” can rarely be measured satisfactorily, 522. — A modification of the second type of study gives better results, 524. — The two types compared, 525. — Relation between theoretical and empirical demand curves, 526. — Three divergent statistical results, 529. — Determining the true relationship, 536.