Occurrence Exceedance Probability
Definition
Occurrence exceedance probability (OEP) is the modeled annual probability that the largest single event in a year produces losses above a given amount. Plotted across loss levels, it forms the OEP curve. It is the right measure for per-occurrence covers, because those pay on one event at a time; the attachment probability of an occurrence layer is the OEP at its attachment point.
Why it matters
Reading an ILS offering, the probabilities quoted for an occurrence layer come from the OEP curve. Knowing that tells an investor what is and is not counted: a year of several moderate events can leave the OEP-based attachment probability untouched while still eroding an aggregate layer.
Common misconceptions
- •OEP does not measure total annual losses; it measures the largest event.
- •A 1% OEP loss is not a once-a-century certainty.
- •OEP curves differ by model and exposure date.
- •The label is only a starting point. Investor outcomes usually depend on the governing documents, cash-flow priority, valuation policy, disclosure quality, and manager discretion surrounding the term, not just the shorthand definition used in marketing materials.
Technical details
Relations
Attachment probability (occurrence layer) = OEP at the attachment.
Exhaustion probability = OEP at the exhaustion point.
Return period = 1 / OEP.
How investors apply the term
Investors usually apply this term by tying the definition back to the controlling documents and then asking which cash flows, rights, restrictions, or valuation assumptions it changes. A useful review looks at the term alongside fees, leverage, liquidity, reporting frequency, and manager discretion rather than treating it as a standalone concept.
The practical diligence test is simple: identify who benefits if the assumption is wrong, who has control when conditions change, and what evidence the investor receives after closing. Terms that cannot be reconciled to documents, cash movements, and monitoring data should be treated as marketing shorthand until proven otherwise.
Frequently asked questions
- How is an OEP curve built?
- By simulating many years of events with a catastrophe model and recording each year's largest event loss. Swiss Re's 2011 primer describes the result as an exceedance probability curve that 'provides estimates of expected annual losses and the attachment probabilities for different loss levels.'
- How does it relate to a probable maximum loss?
- A 1-in-100 probable maximum loss is the loss at 1% exceedance probability. Allstate reported a modeled 1-in-100 probable maximum loss of about $3.1 billion net of reinsurance at December 31, 2025, without stating whether it is an occurrence or aggregate measure.
- When should I use OEP rather than AEP?
- For per-occurrence layers and most cat bonds with per-event triggers. Aggregate layers need the aggregate curve.