Date of Degree


Document Type


Degree Name





Cynthia Calkins

Committee Members

Elizabeth Jeglic

William Gottdiener

Erika Carr

Dale Sebastian

Subject Categories

Clinical Psychology | Cognitive Psychology | Psychology


placebo, placebo effect, placebo response, randomization ratio, unbalanced randomization ratio


Placebos are commonly employed in clinical trials as inactive treatments to which experimental treatments are compared against in order to control for psychological “noise.” Randomized double-blind placebo control studies are considered the “gold standard” in epidemiologic research because they can provide the strongest possible evidence of causation if designed correctly (Hulley, Cummings, Browner, & Grady, 2007). One phenomenon that poses a threat to the integrity of this evidence is the placebo response (PR), or referred to as the “placebo effect.” Expectancy is considered a central PR mechanism and boasts the most empirical support among all proposed mechanisms. Expectancy is not limited to treatment efficacy. Experimental variables also affect expectancy and subsequent PR. Unlike standard clinical care, there is no guarantee that subjects in a randomized placebo control trial will receive experimental treatment. This uncertainty affects expectancy and subsequent PR. The randomization ratio (RR) represents the distribution of subjects across conditions and thus the probability of receiving either experimental or placebo treatment. The RR of 1:1 (50-50 chance of being randomized to experimental or placebo condition; maximum uncertainty) has long been considered the norm, but an increasing number of trials have begun employing unbalanced RR. Studies generally agree that larger allocation to experimental treatment (and smaller allocation to placebo) results in increased PR. However, there are many limitations in the literature that make reaching this conclusion tentative. Objectives: The purpose of the current study was to determine whether RR affects the PR. Methods: The experiment included eight groups with varying randomized ratios and employed a performance-based measure that was successfully enhanced by a placebo treatment. Conclusion: The results of this study provides support for the theory that there exists a relationship between the randomization ratio and the magnitude of the placebo response.