The demand for a product summarizes the relationship between the quantity of the good or service desired and the price at which it is offered for sale. Besides the price of the good, many factors such as the prices of related goods, changing tastes and preferences, consumer income, or expectations about the future influence the demand for a particular product. The elasticity of demand is a measure of the responsiveness of product demand to changes in one of these determinants. The demand determinants for which elasticity measures are typically computed are the price of the good or service, the income of the consumer, and the prices of related goods or services. Elasticity measures are particularly useful because they focus on the relative magnitudes of changes rather than the absolute. As such, elasticity measures are free of units of measurement. This characteristic makes them particularly useful for comparing demand responses across products, countries, and individuals.