Combining risks in estimating

A method is described which attempts to provide a practical way of building up an estimate which will be exceeded with a calculated probability. The stakes are high in a large project but a statistically rigorous method would be impossibly complex for projects with a large number of risks to consider. The method described is a compromise between precision and practicability. It follows the spirit of a full probability treatment but is appropriate to the level of reliability in the subjective evaluations of risk which form a large part of the input to any method of risk analysis.