Technical Reserves and Claims Development Triangles

When an insurer writes a policy, most of the eventual cost isn't known for years: a claim may take a long time to be reported, and once reported, it can take even longer to fully settle, especially for liability lines like auto injury or medical malpractice where legal disputes and slow-developing injuries stretch out payment. Technical reserves are the balance-sheet liability an insurer holds to cover everything it expects to eventually pay on claims from policies already written, whether those claims have been reported yet or not.

The standard tool for estimating this is a claims development triangle: rows are the "accident year" (when the loss occurred), columns are the "development year" (how many years since the loss, claims have been paid or reported), and each cell shows cumulative claims paid or reported by that combination. Because more recent accident years haven't had as much time to develop, the triangle has data only on and above its diagonal — the bottom-right is unfilled future development that must be projected. The chain-ladder method fills in that missing corner by applying development factors observed in older, fully-matured accident years to the still-developing recent years, producing an estimate of each year's ultimate claims cost and, by subtracting what's already been paid, the reserve still owed.

Reserving errors compound quietly: an insurer that consistently under-reserves looks more profitable than it is until the claims eventually mature and the shortfall is recognized, which is why reserve adequacy is one of the first things analysts and regulators scrutinize in an insurer's financials.