Since 2002, the Central Population Register (ZMR) has formed the data basis for recording the population status in Austria. The results are available for the beginning of each quarter (reference dates 1 January, 1 April, 1 July and 1 October), with a more detailed breakdown at the beginning of each year. The number of inhabitants on the respective reference date includes persons, who have been registered with a main residence in Austria for more than 90 days around the reference date. Before 2002, population figures were enumerated by censuses. For the inter-censal period annual estimations were calculated of the population at the beginning of the year.