Macroeconomics is one of the subdivisions in the field of economics that studies the behavior and performance of an economy as a whole. It emphasize on the aggregate changes in the economy such as unemployment, growth rate, gross domestic product and inflation. Microeconomics stands in the contrast to macroeconomics, which involves in "The sum total of economic activity, dealing with the issues of growth, inflation, and unemployment and with national policies relating to these issues"; Supply and demand is an economic model of the price determination in a perfectly competitive market. It concludes that in a perfectly competitive market with no externalities, per unit taxes, or price controls, the unit price for a particular good is the price at which the quantity demanded by consumers equals the quantity supplied by producers. This price results in a stable economic equilibrium.