Incurred But Not Reported (IBNR) Estimation Methods
A technical overview of the primary methods actuaries use to estimate IBNR reserves.
What Is IBNR?
IBNR (Incurred But Not Reported) represents claims that have occurred but have not yet been reported to the insurer. This includes both pure IBNR (truly unreported claims) and development on known claims (case reserve inadequacy). IBNR is often the largest and most uncertain component of an insurer's total reserves, particularly for long-tail lines where reporting delays can span years.
Common Estimation Methods
The chain ladder (or link ratio) method develops paid or incurred losses to ultimate using historical development patterns. The Bornhuetter-Ferguson method blends actual experience with an a priori expected loss ratio, providing stability when early development data is sparse. The Cape Cod method is a variant that uses earned premium to weight the expected loss ratio. Frequency-severity approaches estimate IBNR by separately projecting unreported claim counts and their expected average cost. Each method has strengths and weaknesses, and actuaries often use multiple methods to triangulate a best estimate.