At its simplest, a risk model is comprised of a set of inputs and outputs related in some way. By specifying probability distributions on the inputs and applying the model relationship, you can find the probability distributions of the outputs. In the previous three chapters we have discussed three types of models:

reliability models where the inputs and outputs are binary – the system and its components either work or do not; cost models where, at their simplest, the output is the sum of the inputs; project schedule models where, again at their simplest, the output is the duration of the project determined through the duration, resourcing and interrelationships of a number of tasks.