Short-term marginal cost refers to the total cost increase caused by each additional unit output of the manufacturer in a short period of time.

Short-term marginal cost is expressed by SMC; Δ q represents increased yield; Taking STC as the increment of total cost, then:

Smc = δ STC/δ q or SMC=dSTC/dQ.

Marginal propensity to consume (MPC): the ratio of the change of consumption to the change of income, that is, the change of consumption per kloc-0/unit income, which is expressed by the formula: MPC =δC/δY .(δC refers to the change of consumption and δ Y refers to the change of income).