Definition: Parallelogram with horizontal 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 had a given age a in completed years (age last birthday) when they experienced that demographic event. The denominator is the arithmetic mean of the persons who reached the age a in completed years during the calendar year t+a and the number of persons who reached the age a+1 during the calendar year t+a+1. The demographic event under study may take place in calendar year t+a or t+a+1.
Eurostat, "Demographic statistics: Definitions and methods of collection in 31 European countries (2003 Edition)", Luxembourg, 2003