Category: Claims handling · Reviewed by Taylor Watts, Broker · New Business · Last reviewed 2026-06-11
IBNR (Incurred But Not Reported) reserves are the actuarial provisions set aside for claims that have occurred during the policy period but have not yet been reported to the insurer.
Every insurance portfolio carries a tail of unreported claims at any reporting date. Some have happened but not yet been discovered. Some have been discovered but not yet notified to the insurer. Some have been notified but not yet entered in the system. The IBNR reserve quantifies the insurer’s best estimate of the cost of all these unreported claims, plus the future cost of claims already reported but not yet entered with a case reserve.
IBNR is distinct from case reserves (set claim by claim on reported claims) and from IBNER (the expected further development of reported claims). IBNR is a portfolio-level construct calculated by the actuarial function rather than the claims function. It is one of the most important figures on any insurer’s balance sheet and one of the most difficult to estimate, because the actuary must project the unknown by reference to the known.
Solvency II technical provisions (Solvency II Directive 2009/138/EC, Articles 76 to 86, as retained in UK law and reflected in the PRA Rulebook) require insurers to hold provisions representing the probability-weighted average of future cash flows for all claims liabilities, including those not yet reported. IBNR is the practical embodiment of that requirement at the claims-liability level.
The PRA’s Supervisory Statement SS5/14 sets out expectations on reserving methodology, including the use of multiple methods, the documentation of assumptions, the role of the actuarial function and the engagement of the board. The actuarial function (Article 48) is responsible for the appropriateness of IBNR methodologies and for an annual report to the board.
Under IFRS 17, IBNR forms part of the fulfilment cash flows that underlie the measurement of insurance contract liabilities. The boundaries of the contract, the discount rate, the risk adjustment for non-financial risk and the contractual service margin all depend on IBNR estimates being well-grounded.
For Lloyd’s syndicates, Lloyd’s prescribes additional discipline through the QMA and QMB returns and through Lloyd’s reserving oversight. IBNR is reported at the syndicate-year and class level, with central market benchmarks providing the basis for supervisor challenge.
IBNR is calculated by actuarial methods applied to claim-development triangles. The basic input is a triangle of paid losses (or incurred losses, defined as paid plus case reserves) by accident year (or underwriting year) and development year. The actuary applies one or more methods to project the ultimate.
The Chain Ladder method projects ultimate losses by applying age-to-ultimate factors to the most recent diagonal of the triangle. The factors are derived from historic development. Chain Ladder is simple, transparent and well-understood, but it is vulnerable to distortion in immature years where a single large claim can throw the factors out.
The Bornhuetter-Ferguson method blends an a priori expected loss ratio with observed development. For immature years (the most recent two or three) this method tends to be more stable. It is often preferred for long-tail lines.
The Cape Cod method derives the a priori expected loss ratio from observed data, removing the subjectivity of the BF approach. The Mack method extends Chain Ladder with a stochastic framework allowing confidence intervals around the estimate.
In practice, actuaries run multiple methods in parallel, compare the results, and select an estimate that reflects the strengths and weaknesses of each method in context. Adjustments are made for known emerging issues — claim inflation running ahead of historic averages, court decisions affecting damages, new latent exposures (such as cladding-related claims emerging in property and PI books from 2017 onwards).
IBNR is split into “pure IBNR” (provisions for claims not yet reported) and “IBNER” (provisions for further development of reported claims). The two are often calculated together but reported separately for analysis and disclosure.
“Negative IBNR” can arise where case reserves are systematically over-strong relative to ultimate cost; the IBNR adjustment then reduces the aggregate reserve. This is not pathological — it can be a sign of conservative case reserving — but persistent negative IBNR may indicate that the case-reserve methodology is too prudent.
“Catastrophe IBNR” is a distinct category. After a major event, the actuary projects total ultimate event cost using exposure data, claim-frequency models and PML estimates; the IBNR is set as the projected event cost less the case reserves on claims notified to date. Catastrophe IBNR runs off rapidly as claims notify.
“Latent IBNR” applies to long-tail exposures with very long reporting periods — asbestos, environmental, child abuse compensation, cladding claims under the Building Safety Act 2022 extended limitation periods. Latent IBNR is among the hardest reserves to estimate and is heavily scrutinised by auditors and supervisors.
“Reinsurance IBNR” is the recoverable element of IBNR — the actuary’s estimate of the proportion of gross IBNR that will be recovered from reinsurers. It is netted against gross IBNR to derive net IBNR and is subject to credit-quality adjustments for non-performance risk.
A specialty insurer’s casualty book at 30 June 2026 has the following structure. Paid losses to date £127m. Case reserves £64m. Total incurred to date £191m. The actuary’s Chain Ladder projection on paid losses gives an ultimate of £241m. Chain Ladder on incurred losses gives £236m. BF on the 2024 and 2025 underwriting years (immature) blends an a priori loss ratio of 62% with observed development to give an ultimate of £232m. The actuary selects an ultimate of £237m, weighted across methods. Gross IBNR is therefore £237m – £191m = £46m. The reinsurance programme is expected to recover 24% of IBNR on the proportional treaty; net IBNR is £46m × (1 – 0.24) = £35m. The actuary documents the methods, the selection rationale and the sensitivity of the result to alternative assumptions in the formal IBNR memo.
By Matt Bartlett, Director, on 2026-06-11. Next review: 2026-12-11.
This entry is part of the Apex Insurance Wiki. Last reviewed by Matt Bartlett on 2026-06-11. Apex Insurance Brokers Limited, FCA FRN 724952, Companies House 07014570. Not regulated advice — consult your broker on your specific position.
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