This excerpt taken from the PGR 8-K filed Jun 30, 2008.
Section VI Terms and Definitions
Accident period losses: Losses for each accident are assigned to the period in which the accident occurred. Accident periods used in our analysis are generally three months (accident quarter), six months (accident semester), or twelve months (accident year). Payments and reserve changes, regardless of when they are made, are assigned to that same period in which the accident occurred. Therefore, accident period results will change over time as the losses develop.