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Taking a theory and quantifying it
Economists develop economic models to explain consistently recurring relationships. Their models link one or more economic variables to other economic variables (see "What Are Economic Models," F&D, June 2011). For example, economists connect the amount individuals spend on consumer goods to disposable income and wealth, and expect consumption to increase as disposable income and wealth increase (that is, the relationship is positive).
There are often competing models capable of explaining the same recurring relationship, called an empirical regularity, but few models provide useful clues to the magnitude of the association. Yet this is what matters most to policymakers. When setting monetary policy, for example, central bankers need to know the likely impact of changes in official interest rates on inflation and the growth rate of the economy. It is in cases like this that economists turn to econometrics.
Econometrics uses economic theory, mathematics, and statistical inference to quantify economic phenomena. In other words, it turns theoretical economic models into useful tools for economic policymaking. The objective of econometrics is to convert qualitative statements (such as "the relationship between two or more variables is positive") into quantitative statements (such as "consumption expenditure increases by 95 cents for every one dollar increase in disposable income"). Econometricians-practitioners of econometrics-transform models developed by economic theorists into versions that can be estimated. As Stock and Watson (2007) put it, "econometric methods are used in many branches of economics, including finance, labor economics, macroeconomics, microeconomics, and economic policy." Economic policy decisions are rarely made without econometric analysis to assess their impact.
A daunting task
Certain features of economic data make it challenging for economists to quantify economic models. Unlike researchers in the physical sciences, econometricians are rarely able to conduct controlled experiments in which only one variable is changed and the response of the subject to that change is measured. Instead, econometricians estimate economic relationships using data generated by a complex system of related equations, in which all variables may change at the same time. That raises the question of whether there is even enough information in the data to identify the unknowns in the model.
Econometrics can be divided into theoretical and applied components.