The chief difficulties in the computation of statistical laws of demand are due to changes that occur in the market during the period to which the statistics of prices and of quantities of commodities refer. In order that the statistical laws of demand shall have sufficient validity to serve as prediction formulae, the observations must be numerous; and in order to obtain the requisite number of observations, a considerable period must be covered. This usually means that, during the interval surveyed in the statistical series, important changes occur in the condition of the market. But in case of staple commodities, such as the agricultural products with which we shall have to deal, the effects of those changes in the condition of the market that obscure the relation between prices and amounts of commodity may be largely eliminated. As far as the law of demand is concerned, the principal dynamic effects that need to be considered are changes in the volume of the commodity that arise from the increasing population, and changes in the level of prices which are the combined result of causes specifically responsible for price cycles and of causes that produce a secular trend in prices. The effects of these two fundamental changes may be eliminated approximately by a single statistical device, namely, by deducing the law of demand from a generalized treatment of the elasticity of demand.