The additional cost from an increase in an activity. This is the addition to total cost resulting from a unit increase if it varies discretely, or the addition to total cost per unit of the increase, if it varies continuously. Marginal cost may be short-run, when only some inputs can be changed, or long-run, when all inputs can be adjusted. Marginal private cost is marginal cost falling on the person or firm deciding on the scale of the activity, excluding any external costs; marginal social cost includes external costs as well as private cost falling on the decision-taker.