A measure of the relationship between two variables. Elasticity expresses the percentage change in the variable of interest that is caused by a 1% change in another variable. For example, an income elasticity of +1.3 for unit automobile sales means that a 1% increase in income will lead to an increase of 1.3% in the unit sales of automobiles. It is typically easier to think about elasticities than about marginal propensities (which show the unit change in the dependent variable Y when X is changed by one unit).