Most mineral processing system models include variables that have either systematic errors (bias) or randomness associated with them. It is essential to incorporate these uncertainties when analyze these models. In general, there are a couple of different methods of variance or uncertainty analysis used by analysts, such as sensitivity analysis, Monte Carlo simulation, etc. In this paper, the law of propagation of errors (LPE) and one of the favorite analysis tools, Monte Carlo simulation, are presented. Limitations of the LPE are emphasized. Simple applications of these two methods in mineral processing are also briefly shown, as well as a case study of evaluating the variance of flow rate in a flotation circuit. (C) 2003 Elsevier Science Ltd. All rights reserved.