Health Services Research (HSR) Methods
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Independent Variable
Synonyms: Explanatory Variable; Predictor Variable; Regressor Variable; Regressor; Covariate; Exogenous Variable; Right Hand Variable; Manipulated Variable

A measurement (or multiple measurements) of individuals, populations, organizations or other relevant unit of analysis (possibly experimentally manipulated) that explains variation in the outcome).

In an experiment, the independent variable is the variable that is manipulated in order to determine its relationship to an outcome of interest. In a causal model the cause is the independent variable. Often denoted as X in regression models. In the context of regression equations, the terms "independent" and "explanatory" variables often are used interchangeably. However, explanatory variables need not be independent (of each other) in systems of simultaneous equations. Thus, the term "explanatory variable" is more general than the term "independent variable."

Further Reading
Dowd, B. and R. Town. Does X Really Cause Y? September 2002. Available at http://www.hsrmethods.org/files/FileDownloads/DoesXCauseY.pdf .

Campbell, D. and J. Stanley. 1963. Experimental and Quasi-Experimental Designs for Research, Boston, MA: Houghton Mifflin Company.