A control variable is a factor held constant by a researcher to test the relative impact of an independent variable on a dependent variable. By keeping the control variable the same across different conditions or groups in a study, researchers can better isolate the effect of the independent variable they are interested in. This helps to rule out the possibility that changes in the dependent variable are due to the control variable rather than the independent variable. Using control variables is a key aspect of experimental and quantitative research designs aimed at establishing causal relationships between variables. For example, in a study examining the effect of a new teaching method (independent variable) on student test scores (dependent variable), researchers might control for students' prior academic performance (control variable) to ensure that any observed changes in test scores are indeed due to the new teaching method and not pre-existing differences in academic ability.