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📈 '''Aggregate exceedance probability''' (AEP) is a [[Definition:Catastrophe modeling | catastrophe modeling]] metric that expresses the likelihood that total cumulative losses from all events in a given period — typically one year — will exceed a specified monetary threshold. In the insurance and [[Definition:Reinsurance | reinsurance]] industry, AEP is one of the two principal probability metrics (alongside [[Definition:Occurrence exceedance probability (OEP) | occurrence exceedance probability]], or OEP) used to quantify the [[Definition:Tail risk | tail risk]] embedded in a portfolio exposed to [[Definition:Natural catastrophe | natural catastrophe]] perils such as hurricanes, earthquakes, typhoons, and floods. The critical distinction is that AEP captures the accumulation of losses from multiple events — recognizing that a bad year may involve several moderate catastrophes rather than a single massive one — whereas OEP focuses on the single largest event.

🔬 Producing an AEP curve requires running a portfolio through a [[Definition:Catastrophe model | catastrophe model's]] stochastic event set — often comprising tens of thousands of simulated years, each containing a realistic sequence of potential catastrophe events. For every simulated year, the model calculates the aggregate loss across all events in that year after applying the portfolio's policy terms, [[Definition:Deductible | deductibles]], and applicable [[Definition:Reinsurance program | reinsurance recoveries]]. These simulated annual aggregate losses are then ranked and assigned probabilities, producing a full exceedance probability curve. A statement such as "the 1-in-100-year AEP loss is $500 million" means there is a 1% probability that total annual catastrophe losses across all events will exceed $500 million. Insurers, reinsurers, and [[Definition:Insurance-linked securities (ILS) | ILS]] investors use AEP outputs to set [[Definition:Aggregate limit | aggregate limits]], price [[Definition:Aggregate excess of loss | aggregate excess of loss]] covers, calibrate [[Definition:Probable maximum loss (PML) | PML]] estimates, and satisfy [[Definition:Regulatory capital | regulatory capital]] requirements — including [[Definition:Solvency II | Solvency II's]] [[Definition:Solvency capital requirement (SCR) | SCR]] calculations that explicitly reference catastrophe risk at specified return periods.

🎯 AEP holds particular weight in portfolio management decisions because it captures frequency-driven accumulation risk that OEP alone would miss. Consider a [[Definition:Property catastrophe reinsurance | property catastrophe reinsurer]] operating in a region prone to both earthquakes and windstorms: even if no single event breaches the OEP threshold at a given return period, the combined toll of several smaller events in the same year could generate aggregate losses that severely impair capital. This is precisely the scenario AEP is designed to measure. [[Definition:Rating agency | Rating agencies]] such as [[Definition:AM Best | AM Best]] and [[Definition:S&P Global Ratings | S&P Global Ratings]] evaluate catastrophe-exposed insurers on both AEP and OEP bases, and the growing frequency of secondary-peril events — [[Definition:Convective storm | convective storms]], wildfires, and floods — has elevated the importance of AEP analysis in recent years, since these perils tend to generate high-frequency, moderate-severity losses that aggregate quickly. For [[Definition:Chief risk officer (CRO) | chief risk officers]] and [[Definition:Actuarial science | actuaries]], the AEP curve is indispensable for stress-testing capital adequacy under scenarios where the year delivers not one headline catastrophe but a relentless series of them.

'''Related concepts:'''
{{Div col|colwidth=20em}}
* [[Definition:Occurrence exceedance probability (OEP)]]
* [[Definition:Catastrophe model]]
* [[Definition:Probable maximum loss (PML)]]
* [[Definition:Aggregate excess of loss reinsurance]]
* [[Definition:Insurance-linked securities (ILS)]]
* [[Definition:Return period]]
{{Div col end}}

Definition:Aggregate exceedance probability - Revision history

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