**Definition:**A control variable is a variable that is held constant in a research analysis. The use of control variables 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 variable intervening? 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?