" Demand for a product is affected by the cost and quality, broadly defined, of substitute products. If the cost of a substitute falls in relative terms, or if its ability improves to satisfy the buyer’s needs, industry growth will be adversely affected (and vice versa). Examples are the inroads that television and radio have made on the demand for live concerts by symphony orchestras and other performing groups; the growth in demand for magazine advertising space as television advertising rates climb sharply and prime advertising television time becomes increasingly scarce; and the depressing effect of rising prices on the demand of such products as chocolate candy and soft drinks relative to their substitutes. "

― Michael E. Porter , Competitive Strategy: Techniques for Analyzing Industries and Competitors