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Control Variables
In experimental and observational design and data analysis, the term control variable refers to variables that are not of primary interest (i.e., neither the exposure nor the outcome of interest) and thus constitute an extraneous or third factor whose influence is to be controlled or eliminated. The term refers to the investigator's desire to estimate an effect (such as a measure of association) of interest that is independent of the influence of the extraneous variable and free from bias arising from differences between exposure groups in that third variable.
Extraneous variables of this class are usually those variables described as potential confounders in some disciplines. Controlling for a potential confounder, which is not an effect modifier or mediator, is intended to isolate the effect of the exposure of interest on the outcome of interest while reducing or eliminating potential bias presented by differences in the outcomes observed between exposed and unexposed individuals that are attributable to the potential confounder. Control is achieved when the potential confounder cannot vary between the exposure groups, and thus the observed relationship between the exposure and outcome of interest is independent of the potential confounder.
As an example, if an investigator is interested in studying the rate of a chemical reaction (the outcome) and how it differs with different reagents (the exposures), the investigator may choose to keep the temperature of each reaction constant among the different reagents being studied so that temperature differences could not affect the outcomes.
Potential control variables that are mediators in another association of interest, as well as potential control variables that are involved in a statistical reaction with other variables in the study, are special cases which must be considered separately. This entry discusses the use of control variables during the design and analysis stages of a study.
Design Stage
There are several options for the use of control variables at the design stage. In the example about rates of reaction mentioned earlier, the intention was to draw conclusions, at the end of the series of experiments, regarding the relationship between the reaction rates and the various reagents. If the investigator did not keep the temperature constant among the series of experiments, difference in the rate of reaction found at the conclusion of the study may have had nothing to do with different reagents, but be solely due to differences in temperature or some combination of reagent and temperature. Restricting or specifying a narrow range of values for one or more potential confounders is frequently done in the design stage of the study, taking into consideration several factors, including ease of implementation, convenience, simplified analysis, and expense. A limitation on restriction may be an inability to infer the relationship between the restricted potential confounder and the outcome and exposure. In addition, residual bias may occur, owing to incomplete control (referred to as residual confounding).
Matching is a concept related to restriction. Matching is the process of making the study group and control group similar with regard to potential confounds. Several different methods can be employed, including frequency matching, category matching, individual matching, and caliper matching. As with restriction, the limitations of matching include the inability to draw inferences about the control variable(s). Feasibility can be an issue, given that a large pool of subjects may be required to find matches. In addition, the potential for residual confounding exists.
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