A critical component of how we understand a mental process is given by measuring the effect of varying the workload. The capacity coefficient (Townsend and Nozawa, 1995 and Townsend and Wenger, 2004) is a measure on response times for quantifying changes in performance due to workload. Despite its precise mathematical foundation, until now rigorous statistical tests have been lacking. In this paper, we demonstrate statistical properties of the components of the capacity measure and propose a significance test for comparing the capacity coefficient to a baseline measure or two capacity coefficients to each other.