The annual average population is a central reference figure for calculating various statistical key figures. The annual average population itself is calculated from the population at the beginning of each quarter, whereby it is the arithmetic mean of the four quarterly averages. The formula for calculating the annual average population is: annual average = (population on 1 January + 2 times population on 1 April + 2 times population on 1 July + 2 times population on 1 October + population on 1 January of the following year) divided by 8.