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📊 '''Accident year loss ratio''' is a key [[Definition:Underwriting | underwriting]] performance metric that measures [[Definition:Incurred losses | incurred losses]] attributable to events occurring during a specific twelve-month period, expressed as a percentage of [[Definition:Earned premium | earned premiums]] for that same period. Unlike a [[Definition:Calendar year loss ratio | calendar year loss ratio]], which blends current-period activity with reserve adjustments from prior years, the accident year approach isolates the economic cost of claims arising from a defined exposure period. This distinction makes it one of the most closely watched indicators among [[Definition:Actuarial analysis | actuaries]], [[Definition:Underwriter | underwriters]], and financial analysts evaluating an insurer's true risk selection and pricing adequacy.

⚙️ The calculation begins by assigning every reported and [[Definition:Incurred but not reported (IBNR) | IBNR]] claim to the year in which the underlying loss event took place, regardless of when the claim was actually filed or paid. Those estimated [[Definition:Ultimate loss | ultimate losses]] are then divided by earned premiums for the corresponding period. Because IBNR estimates are inherently uncertain — particularly for [[Definition:Long-tail business | long-tail lines]] such as [[Definition:Liability insurance | liability]], [[Definition:Workers' compensation insurance | workers' compensation]], or [[Definition:Professional indemnity insurance | professional indemnity]] — the accident year loss ratio for a given period will continue to develop over time as claims mature. Analysts track this development pattern closely: favorable development suggests initial [[Definition:Loss reserve | reserves]] were conservative, while adverse development signals that losses were underestimated. Under [[Definition:IFRS 17 | IFRS 17]], the emphasis on current estimates of fulfillment cash flows reinforces the importance of accident-year thinking, while [[Definition:US GAAP | US GAAP]] reporting has long required insurers to disclose incurred loss triangles that facilitate accident year analysis.

💡 Investors and [[Definition:Rating agency | rating agencies]] favor the accident year loss ratio precisely because it strips away the noise introduced by prior-year reserve movements, offering a cleaner read on whether today's [[Definition:Premium | premiums]] are adequate to cover today's losses. A company might post a flattering calendar year result by releasing redundant reserves from earlier periods while quietly deteriorating on an accident year basis — a pattern that, if undetected, can erode [[Definition:Policyholder surplus | surplus]] over multiple cycles. For [[Definition:Reinsurance | reinsurers]] negotiating treaty terms and [[Definition:Cedent | cedents]] evaluating their own portfolio health, accident year metrics provide the analytical foundation for informed pricing decisions and [[Definition:Reserving | reserving]] strategies across virtually every major insurance market.

'''Related concepts:'''
{{Div col|colwidth=20em}}
* [[Definition:Calendar year loss ratio]]
* [[Definition:Loss development]]
* [[Definition:Combined ratio]]
* [[Definition:Incurred but not reported (IBNR)]]
* [[Definition:Loss reserve]]
* [[Definition:Earned premium]]
{{Div col end}}

Definition:Accident year loss ratio - Revision history

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