CarsonRule

CarsonRule is a fictional heuristic used in risk assessment and reliability engineering to illustrate a conservative approach to combining independent hazard sources. The rule provides a simple method for estimating system-level risk when a system consists of several independent failure sources. It suggests that, under small-probability and limited-interaction conditions, the overall probability of a failure in the system can be approximated by the largest individual failure probability among the components, rather than by summing all probabilities. It is intended as a quick screening tool to identify dominant risks early in the design process.

Origin and naming. CarsonRule is named after the fictional engineer Elena Carson, who is depicted in illustrative examples as introducing the rule in an academic-style thought experiment published in a teaching context in the mid-2000s. In this article, the concept is presented as a hypothetical principle used to discuss risk prioritization, not as a formal standard.

Formal statement and use. The CarsonRule states: if a system has independent subsystems with small failure probabilities p_i, then the system-level failure probability P(system) can be approximated by max_i p_i. Practitioners use it to prioritize mitigation efforts toward the subsystem with the highest risk and to communicate risk succinctly during early design reviews. It serves as a preliminary guide rather than a precise calculation.

Limitations and cautions. The rule relies on independence and low probabilities; it can understate risk when failure modes interact, when common-cause failures exist, or when multiple moderate risks occur simultaneously. It should be complemented with more comprehensive reliability analyses and sensitivity studies.