Learning about simulations is one of the first subjects in which an ORSA moves from theory into application. A simulation is merely a representation of a real-world system or process over time. There are several reasons why you would use a simulation, but a key reason to use simulations is that a reasonable calculation or statistical analysis could not happen otherwise. In other words, the system we are analyzing is too complicated to narrow its essence into a series of equations. For military ORSAs, our involvement with simulations usually focuses on some variant of a combat model. Since combat between two forces involves multiple factors, there is not a single simulation capable of replicating this type of complexity. Thus, analyses involving simulations incorporate experts from numerous disciplines such as applied mathematicians, computer scientists, physicists, ORSAs, and other military experts.