Probability

There are two types of events which occur: deterministic and stochastic (or random). A event is deterministic if the same set of inputs always yield the same output. For example, Newtonian physics models deterministic phenomena. Engineering design, on the other hand, necessarily involves probability models since the materials out of which system components are fabricated typically have defects that cannot be controlled completely. For example, airplane components degrade with every takeoff and landing. Changes in temperature and pressure and a myriad of other factors during flight put stress on critical parts. Expensive maintenance procedures can be done only periodically and only if qualified staff are available. These factors, difficult to predict with certainty in advance and therefore considered random effects, must be taken into account in the design process. Failure to do so may result in a catastrophic accident with the loss of many lives.