Saving is a flow and refers to the excess of income over consumption in a given period. Savings are a stock and refer to the quantity of assets held. The stock of savings is accumulated from the flow of saving over a number of periods. The average propensity to save is the ratio of saving to income; the marginal propensity to save is the proportion of any addition to income that is saved. The interest-elasticity of saving measures the proportional response of saving to changes in interest rates. See also contractual savings; planned savings; propensity to save.