Definition: Parallelogram with vertical sides method
Method used to compute demographic rates of a certain demographic event, e.g. death or giving birth. The numerator consists of all persons who were born in a given calendar year t and who experienced the demographic event during the calendar year t+a. The denominator is the arithmetic mean of the persons who were of completed age a-1 on 1 January of the calendar year t and the number of persons who were of completed age a at the end of that calendar year. The demographic event may be experienced by people of age a-1 and by people of age a (age in completed years), depending on whether the event takes place before or after the ath birthday.
Eurostat, "Demographic statistics: Definitions and methods of collection in 31 European countries (2003 Edition)", Luxembourg, 2003