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📋 '''Incurred but not reported (IBNR) reserve''' is an actuarial estimate of the [[Definition:Reserve | reserve]] an [[Definition:Insurance carrier | insurer]] must hold for [[Definition:Claim | claims]] that have already occurred but have not yet been reported to the company. Every insurance portfolio contains a lag between the moment a loss event takes place and the moment the insured files a claim — a delay that can range from days in [[Definition:Auto insurance | auto insurance]] to years or even decades in [[Definition:Long-tail liability | long-tail lines]] such as [[Definition:Liability insurance | liability]], [[Definition:Professional indemnity insurance | professional indemnity]], or [[Definition:Environmental liability insurance | environmental]] coverages. Because these losses are real but invisible to the insurer's claims department, the IBNR reserve exists to ensure the balance sheet reflects a realistic picture of total outstanding obligations, not just those claims sitting in an adjuster's queue.

⚙️ Actuaries estimate IBNR reserves using a suite of quantitative methods, with the choice of technique depending on the line of business, data maturity, and the insurer's reserving philosophy. The [[Definition:Chain-ladder method | chain-ladder method]] — which applies historical [[Definition:Loss development factor | loss development factors]] to known claim data — is among the most widely used, supplemented by the [[Definition:Bornhuetter-Ferguson method | Bornhuetter-Ferguson method]] for less mature accident years where reported data is sparse. More sophisticated approaches employ stochastic and Bayesian models to produce probability distributions around the IBNR estimate, which is increasingly expected by regulators and [[Definition:Rating agency | rating agencies]]. The treatment of IBNR varies across accounting regimes: under [[Definition:Statutory accounting | U.S. statutory accounting]], IBNR is a component of total [[Definition:Loss reserve | loss reserves]] reported to the [[Definition:National Association of Insurance Commissioners (NAIC) | NAIC]]; [[Definition:IFRS 17 | IFRS 17]] subsumes IBNR within the liability for incurred claims measured at a current, probability-weighted estimate plus a [[Definition:Risk adjustment | risk adjustment]]; and [[Definition:Solvency II | Solvency II]] requires best-estimate provisions that inherently incorporate IBNR alongside a [[Definition:Risk margin | risk margin]].

💡 Getting the IBNR reserve right is arguably one of the highest-stakes exercises in insurance finance. An underestimated IBNR leads to [[Definition:Inadequate reserves | inadequate reserves]], flattering current-period profitability but setting the stage for painful future [[Definition:Reserve development | adverse development]] charges. An overestimated IBNR, while more conservative, ties up [[Definition:Capital | capital]] unnecessarily and can suppress reported earnings, affecting shareholder returns and competitive positioning. The challenge intensifies in emerging risk areas — [[Definition:Cyber insurance | cyber liability]], mass tort litigation, and pandemic-related business interruption — where historical development patterns offer limited guidance. External auditors, regulators, and [[Definition:Reinsurer | reinsurers]] all scrutinize IBNR methodologies closely, and independent [[Definition:Actuarial opinion | actuarial reviews]] of IBNR adequacy are a standard component of [[Definition:Due diligence | due diligence]] in [[Definition:Mergers and acquisitions (M&A) | M&A]] transactions and major [[Definition:Reinsurance | reinsurance]] treaties. For these reasons, IBNR reserving discipline is widely considered a litmus test of actuarial competence and management integrity.

'''Related concepts:'''
{{Div col|colwidth=20em}}
* [[Definition:Loss reserve]]
* [[Definition:Chain-ladder method]]
* [[Definition:Bornhuetter-Ferguson method]]
* [[Definition:Reserve development]]
* [[Definition:Loss development factor]]
* [[Definition:Inadequate reserves]]
{{Div col end}}

Definition:Incurred but not reported (IBNR) reserve - Revision history

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