In health economics, a measure of the sensitivity of demand for a product or service to changes in its price (price elasticity) or the income of the people demanding the product or service (income elasticity). Price elasticity is the ratio of the resulting percentage change in demand to a given percentage change in price. Price elasticity of demand for health services allows one to predict the effect on demand of different cost sharing provisions in proposed NHI programs and thus aids in predicting the differing stress their enactment would place on the health system.