The marginal propensity to import (MPM) is the fractional change in import expenditure that occurs with a change in GDP. Mathematically, the marginal propensity to import (MPM) function is expressed as the ratio of the import (M) function with respect to GDP (Y).<blockquote></blockquote>In other words, the marginal propensity to import is measured as the ratio of the change in imports to the change in GDP, thus giving us a figure between 0 and 1.