To establish an unambiguous causal relationship between the independent and dependent variables, it is necessary to eliminate the possible influence of a confounding variable. Extraneous variables become confounds when they change systematically along with the independent variable. After identifying a short list of extraneous variables that have the potential to become confound- ing variables, it is possible to actively or passively control these variables. The two standard methods of active control are (1) holding a variable constant, and (2) matching values across the treatment conditions. The method for passive control is to randomize these variables across the treatment conditions.