Variation is the root of simulation. OPS allows people to understand the effects of variation. Traditionally distribution fitting is an integral part of simulation, but with OPS this work is done for you. OPS accepts values (e.g. "6"), distributions (e.g. "normal(10,2)"), or, most uniquely, a list of observations (e.g. "6,11,7,7,8,10,15"). If a list of observations is given, then OPS creates a distribution from those observations and samples from it to produce the variability that was observed in the system.