Methodological Urban Legends: The Misuse of Statistical Control Variables

The automatic or blind inclusion of control variables in multiple regression and other analyses, intended to purify observed relationships among variables of interest, is widespread and can be considered an example of practice based on a methodological urban legend. Inclusion of such variables in most cases implicitly assumes that the control variables are somehow either contaminating the measurement of the variables of interest or affecting the underlying constructs, thus distorting observed relationships among them. There are, however, a number of alternative mechanisms that would produce the same statistical results, thus throwing into question whether inclusion of control variables has led to more or less accurate interpretation of results. The authors propose that researchers should be explicit rather than implicit regarding the role of control variables and match hypotheses precisely to both the choice of variables and the choice of analyses. The authors further propose that researchers avoid testing models in which demographic variables serve as proxies for variables that are of real theoretical interest in their data.