Definitions and taxonomy for High Impact Low Probability (HILP) and outlier events

This study outlines a comprehensive effort to clarify and standardise how High Impact Low Probability (HILP) and outlier events are defined and categorised, with the aim of strengthening disaster risk reduction across sectors. It responds to growing recognition that climate‑driven extremes, cascading failures, and systemic shocks increasingly exceed the assumptions of traditional risk assessment. The study explains why existing definitions are fragmented, how the authors conducted a literature review and practitioner validation process, and for whom the resulting academic and operational definitions are intended. It also presents a taxonomy designed to help governments, critical infrastructure operators, and private‑sector actors identify HILPs more consistently, integrate them into risk registers, and improve scenario building, stress testing, and preparedness planning. 

The study recommends adopting harmonised definitions and taxonomies to improve cross‑sectoral communication, strengthen resilience analysis, and support more adaptive, risk‑agnostic planning. It highlights the need for innovative methodologies—such as lateral thinking, counterfactual analysis, and scenario stress testing—to overcome the limitations of lessons learned from past events. The authors emphasise that organisations should focus on critical service continuity, embed HILP considerations into legislation and standards, and design training and tabletop exercises that reflect complex, cascading dynamics. By doing so, policymakers and practitioners can better anticipate emerging risks, allocate resources proportionally to context, and enhance preparedness for future outlier events.