Common random numbers (CRN) is a general variance reducing technique for comparing stochastic models via simulations. By inducing positive correlation between different simulations, CRN are likely to reduce the experimental or sampling variance of the difference between the simulations. However, in this paper my motivation for and interpretations of CRN go beyond the statistical design of a simulation experiment, and into the core of economic and econometric reasoning. In a counterfactual-, scenario- or policy analysis based on stochastic simulation, CRN implement the qualifier in partial analysis known as the ceteris paribus clause. In addition, CRN are consistent with the requirement of super exogeneity for valid policy analysis.