Simulation typically is a deterministic process. For a given set of input variables, the corresponding outputs are always the same. However in the real world, input variables may vary a bit as a result of manufacturing tolerances and geometric properties, for example. Due to the variability of the input variables, an output usually isn't a "fixed" value but rather potentially different values distributed around the mean value.
Input variability is the source of unexpected and often unintended product behavior. Even at world-class manufacturers, it may occur that a design successfully passes deterministic simulation, while some of manufactured items fail production quality control. To avoid this problem in the design stage, the robust design optimization software Optimus takes into account the variability of the design variables, and subsequently applies robustness and reliability concepts and methods to ensure a robust and reliable design. When bringing to market robust designs, the number of product recalls and repairs drops spectacularly, resulting in major savings and a better market reputation.