Loss development

Reviewed by Apex Insurance Brokers · Last reviewed 2026-06-22

Reviewed by Apex Insurance Brokers · Last reviewed 2026-06-22

Reviewed by Apex Insurance Brokers · Last reviewed 2026-06-22

Category: Actuarial fundamentals · Reviewed by Tim Roche, Director · PI & Commercial · Last reviewed

Loss development is the change in the recorded value of claims for a given accident, underwriting or report year as more information becomes available over time. Reported losses initially understate ultimate losses because of:

• IBNR (claims not yet reported).
• IBNER (claims reported but with case reserves below ultimate cost).
• Late reopening of closed claims.
• Litigation outcomes producing settlement at multiples of the case reserve.

Loss development factors (LDFs)

LDFs are multipliers applied to losses at a particular development age (e.g. 12 months) to project them to ultimate. A typical pattern for long-tail casualty might be:

Factors are derived from historical loss triangles, usually by the chain-ladder method.

Long-tail vs short-tail

• Short-tail lines (property, motor own damage) reach ultimate in 1–3 years.
• Long-tail lines (EL, PI, motor BI, casualty reinsurance) can take 10+ years.
• Latent classes (asbestos, pollution, abuse) can develop over decades.

References

• Mack, T. (1993). Distribution-free calculation of the standard error of chain ladder reserve estimates. ASTIN Bulletin 23(2).
• IFoA. General insurance reserving practice.
• Schmidt, K.D. and Wünsche, A. (1998). Chain ladder, marginal sum and maximum likelihood estimation.

Cross-references

• Loss triangle
• Chain ladder method
• IBNR
• IBNER

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