<p><b>Definition: </b>A control variable is a variable that is held constant in a research analysis. The use of <a href="https://www.thoughtco.com/principle-of-optimality-definition-1147078" data-inlink="dBD-yTexKnKtlNE0F9eIDA&#61;&#61;" data-component="link" data-source="inlineLink" data-type="internalLink" data-ordinal="1">control variables</a> is generally done to answer four basic kinds of questions: 1. Is an observed relationship between two variables just a statistical accident? 2. If one variable has a causal effect on another, is this effect a direct one or is it indirect with other <a href="https://www.thoughtco.com/intervening-variable-3026367" data-inlink="8dvGPK2c45JpS4CGUDM7fQ&#61;&#61;" data-component="link" data-source="inlineLink" data-type="internalLink" data-ordinal="2">variable intervening</a>? 3. If several variables all have causal effects on the dependent variable, how does the strength of those effects vary? 4. Does a particular relationship between two variables look the same under various conditions?</p>