Econometrics is an application of Statistics to economic data in order to give empirical content to economic relationships.M. Hashem Pesaran (1987). "Econometrics", , v. 2, p. 8 pp.. Reprinted in J. Eatwell et al., eds. (1990). Econometrics: The New Palgrave, p. 1 pp.. Abstract (2008 revision by J. Geweke, J. Horowitz, and H. P. Pesaran). More precisely, it is "the quantitative analysis of actual economic Phenomenon based on the concurrent development of theory and observation, related by appropriate methods of inference."Paul Samuelson, T. C. Koopmans, and Richard Stone (1954). "Report of the Evaluative Committee for Econometrica", Econometrica 22(2), p. 142. p-146], as described and cited in Pesaran (1987) above. An introductory economics textbook describes econometrics as allowing economists "to sift through mountains of data to extract simple relationships."Paul A. Samuelson and William D. Nordhaus, 2004. Economics. 18th ed., McGraw-Hill, p. 5. Jan Tinbergen is one of the two founding fathers of econometrics.Magnus, Jan & Mary S. Morgan (1987) The ET Interview: Professor J. Tinbergen in: 'Econometric Theory 3, 1987, 117–142.Willlekens, Frans (2008) International Migration in Europe: Data, Models and Estimates. New Jersey. John Wiley & Sons: 117. The other, Ragnar Frisch, also coined the term in the sense in which it is used today.• H. P. Pesaran (1990), "Econometrics", Econometrics: The New Palgrave, p. 2 , citing Ragnar Frisch (1936), "A Note on the Term 'Econometrics'", Econometrica, 4(1), p. 95.
Aris Spanos (2008), "statistics and economics", The New Palgrave Dictionary of Economics, 2nd Edition. Abstract.
A basic tool for econometrics is the multiple linear regression model. Econometric theory uses statistical theory and mathematical statistics to evaluate and develop econometric methods. Econometricians try to find that have desirable statistical properties including unbiasedness, efficiency, and consistency. Applied econometrics uses theoretical econometrics and real-world economic data for assessing economic theories, developing econometric models, analysing economic history, and forecasting.
For example, consider Okun's law, which relates GDP growth to the unemployment rate. This relationship is represented in a linear regression where the change in unemployment rate () is a function of an intercept (), a given value of GDP growth multiplied by a slope coefficient and an error term, :
The unknown parameters and can be estimated. Here is estimated to be 0.83 and is estimated to be -1.77. This means that if GDP growth increased by one percentage point, the unemployment rate would be predicted to drop by 1.77 * 1 points, Ceteris paribus. The model could then be tested for statistical significance as to whether an increase in GDP growth is associated with a decrease in the unemployment, as hypothesized. If the estimate of were not significantly different from 0, the test would fail to find evidence that changes in the growth rate and unemployment rate were related. The variance in a prediction of the dependent variable (unemployment) as a function of the independent variable (GDP growth) is given in polynomial least squares.
Econometrics uses standard statistical models to study economic questions, but most often these are based on observational data, rather than data from experiment. In this, the design of observational studies in econometrics is similar to the design of studies in other observational disciplines, such as astronomy, epidemiology, sociology and political science. Analysis of data from an observational study is guided by the study protocol, although exploratory data analysis may be useful for generating new hypotheses.Herman Wold (1969). "Econometrics as Pioneering in Nonexperimental Model Building", Econometrica, 37(3), pp. 369 -381. Economics often analyses systems of equations and inequalities, such as supply and demand hypothesized to be in equilibrium. Consequently, the field of econometrics has developed methods for identification and estimation of simultaneous equations models. These methods are analogous to methods used in other areas of science, such as the field of system identification in systems analysis and control theory. Such methods may allow researchers to estimate models and investigate their empirical consequences, without directly manipulating the system.
In the absence of evidence from controlled experiments, econometricians often seek illuminating natural experiments or apply Quasi-experiment to draw credible causal inference. The methods include regression discontinuity design, instrumental variables, and difference-in-differences.
This example assumes that the natural logarithm of a person's wage is a linear function of the number of years of education that person has acquired. The parameter measures the increase in the natural log of the wage attributable to one more year of education. The term is a random variable representing all other factors that may have direct influence on wage. The econometric goal is to estimate the parameters, under specific assumptions about the random variable . For example, if is uncorrelated with years of education, then the equation can be estimated with ordinary least squares.
If the researcher could randomly assign people to different levels of education, the data set thus generated would allow estimation of the effect of changes in years of education on wages. In reality, those experiments cannot be conducted. Instead, the econometrician observes the years of education of and the wages paid to people who differ along many dimensions. Given this kind of data, the estimated coefficient on years of education in the equation above reflects both the effect of education on wages and the effect of other variables on wages, if those other variables were correlated with education. For example, people born in certain places may have higher wages and higher levels of education. Unless the econometrician controls for place of birth in the above equation, the effect of birthplace on wages may be falsely attributed to the effect of education on wages.
The most obvious way to control for birthplace is to include a measure of the effect of birthplace in the equation above. Exclusion of birthplace, together with the assumption that is uncorrelated with education produces a misspecified model. Another technique is to include in the equation additional set of measured covariates which are not instrumental variables, yet render identifiable. An overview of econometric methods used to study this problem were provided by David Card (1999).
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In some cases, economic variables cannot be experimentally manipulated as treatments randomly assigned to subjects. In such cases, economists rely on observational studies, often using data sets with many strongly associated , resulting in enormous numbers of models with similar explanatory ability but different covariates and regression estimates. Regarding the plurality of models compatible with observational data-sets, Edward Leamer urged that "professionals ... properly withhold belief until an inference can be shown to be adequately insensitive to the choice of assumptions".
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