Economics abounds with propensities to do various things: consume, save, invest, import, and so on. In each case, it is important to distinguish between the average propensity and the marginal one. The average propensity to consume is simply total consumption divided by total income. The marginal propensity to consume measures how much of each extra dollar of income is consumed: the percentage change in consumption divided by the percentage change in income. The value of the marginal propensity to consume, which determines the multiplier, is harder to predict than the value of the average propensity to consume.