What is the purpose of including a control variable in a study?

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Controlling for other factors is about reducing confounding so you can estimate the treatment’s effect more accurately. A control variable is added because some variables can influence the outcome and are also related to the treatment. By accounting for these factors, you hold them constant in the analysis, which helps ensure that any observed difference in the outcome is more likely due to the treatment itself rather than other influences. For example, in a study of a new teaching method, prior knowledge or baseline ability can affect both which method is used and how students perform; including those as control variables helps isolate the method’s true impact. This isn’t about increasing sample size or about testing whether the data are normally distributed; it’s about reducing bias from confounding to better infer causality.