The Economist's Dictionary of Economics defines Macroeconomics as "The study of whole economic systems aggregating over the functioning of individual economic units. It is primarily concerned with variables which follow systematic and predictable paths of behaviour and can be analyzed independently of the decisions of the many agents who determine their level. More specifically, it is a study of national economies and the determination of national income." Macroeconomists study aggregated indicators such as GDP, unemployment rates, and price indices to understand how the whole economy functions. Macroeconomists develop models that explain the relationship between such factors as national income, output, consumption, unemployment.