Allison D B
Columbia University College of Physicians and Surgeons, New York, USA.
J Consult Clin Psychol. 1995 Jun;63(3):339-43. doi: 10.1037//0022-006x.63.3.339.
In a randomized clinical trial, an experimenter can increase statistical power by including a covariate (e.g., a pretest). This will generally reduce the total number of participants needed to achieve a specified level of power. However, it will also increase the cost per participant. Thus, the question arises, "When do the savings incurred by needing fewer participants exceed the costs incurred by measuring each participant on the covariate?" A simple closed form expression is derived that applied researchers can use in the design phase of studies to answer this question.
