accident year experience
Simply stated, it is the statistical matching of all losses occurring (regardless of when the losses are reported) during a given 12-month period of time, with all premium earned (regardless of when the premium was written) during the same period of time. Calendar year experience, on the other hand, is the statistical matching of all losses incurred (not necessarily occurring) within a given 12-month period, usually beginning on January 1, with all premium earned within the same period of time. Yet a third related term is policy year experience which, to put it simply, is the statistical segregation of all premiums and losses attributable to policies having an inception or renewal date within a given 12-month period.
(See calendar year experience and policy year experience.)