Marginal costing is a type of flexible standard costing that separates fixed costs from variable costs. It is a comprehensive method for planning and monitoring costs based on resource drivers. Marginal costing ensures cost fluctuations from changes in operating levels are accurately predicted and incorporated into variance analysis. It has become widely accepted in business over the last 50 years. Marginal cost is the change in total cost from producing one more unit. It includes any additional costs to produce the next unit and varies depending on production levels and time periods considered. The relationship between marginal cost and economies of scale depends on whether average or marginal costs are falling or rising with production. Externalities can cause private and social costs to diverge.