Adjustment begins when planned purchases and sales differ at the prevailing price. Excess demand indicates that consumers want to buy more than producers offer, while excess supply indicates that producers offer more than consumers want to buy. Price changes respond to these imbalances, moving the market toward the point where intended buying and selling quantities coincide.
A perfectly competitive market contains numerous buyers and sellers, so individual participants treat the market price as given rather than setting it independently. Their separate purchasing and production decisions still influence total demand and supply. The interaction of these decentralized choices allows the market price to coordinate resource allocation across participants.
A change in market conditions alters either the quantity consumers want to buy, the quantity producers want to sell, or both. In a demand-and-supply diagram, the relevant curve changes, producing a new intersection. Comparing the original and revised intersections shows how the equilibrium price and quantity respond to changing conditions.
Consumer surplus and producer surplus provide measures for evaluating the gains associated with market exchange. Together with the equilibrium allocation, they help microeconomists examine how benefits are distributed between buyers and sellers. These measures also support efficiency analysis by showing how an outcome uses available resources relative to alternative market arrangements.
An analysis typically identifies the demand curve, the supply curve, and their intersection. The intersection supplies the equilibrium price and quantity, which can then be compared with a chosen market price to identify excess demand or excess supply. Economists may also calculate or assess consumer and producer surplus to interpret the outcome.
The framework provides a benchmark for comparing an undisturbed market outcome with one affected by a policy. Economists examine how a tax, subsidy, or price control changes the relationship between demand and supply, then assess the resulting price, quantity, resource allocation, and surplus. This comparison clarifies how policy interventions alter market outcomes.