A researcher can operationalize conceptual variables in many different ways. Decisions on this matter are among the most critical that the researcher has to make. This chapter examines how these decisions can affect the internal and the external validity of the experiment. It deals with manipulations of independent variables. The chapter focuses on problems related to the measurement of dependent variables. Ideally, the independent variable is the only variable that varies systematically from one experimental condition to another. In practice, an experimental manipulation almost always includes variables besides those the researcher intended. Researchers often disagree about what constitutes the most appropriate operational definition of a conceptual variable. To operationalize the conceptual variable of anxiety he manipulated subjects’ expectations about the severity of electric shocks they were going to receive. S. Schachter thought the conceptual variable he had operationalized was anxiety but I. Sarnoff and P. Zimbardo thought that it was fear.