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The trick is knowing when

December 1, 2003

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Patrick Corr, chief demographer with the Australian Bureau of Statistics (ABS), explains his numbers game:

Each day Australia's population changes, because of births, deaths and international migration. Tracking the size of the resident population is one of the tasks of the ABS.

This year the ABS has used the latest estimates of the population and an assumed growth rate of one person every 2 minutes and four seconds to project that we will reach 20 million on December 4.

Of course, the number of births and deaths is different every day. Based on a 10-year average, the lowest birth day of the year is December 25, with 374 babies making their appearance. Nine months later, there's a high of 756 babies on September 27.

Each year, more people die in the colder months than in summer. Travellers leave and reach Australia mostly to plane flight schedules, which can be late. So trying to predict the exact time the population will reach 20 million (or who it will be) is not statistically valid. We think we are close in choosing December 4 as the 20 million point.

To make our estimate, we start with the population census, which is like a snapshot of Australia's population every 5 years. Last census (August 7, 2001), some 32,000 temporary collectors were involved in delivering and retrieving forms at every place of residence in Australia. We made special arrangements to count people in hotels and hospitals, homeless people, travellers and Aboriginal and Torres Strait Islander people living in remote communities. We also take into account people who may have been missed in the census or were temporarily overseas on the night. The ABS updates the census-based population estimate every three months.

Population projections help future planning and a website Population Clock provides an estimate each day of Australia's current population. The projections and the clock are the result of formulas using a recent population estimate and assumptions for future population growth. Of course, over time, there is a risk that the actual population growth may be higher or lower than the assumptions used.

Economic, social and political conditions can affect population growth. In the 1930s, with the Depression, it was thought that Australia's population would not grow beyond about 9.5 million. But then came the postwar migration program, and the baby boom which continued through to the mid-1960s.

In 1969, the population was projected to grow to 21.7 million in 2001. In 1978 this was revised down to 18.1 million, with lower migration and fertility rates. Today we believe the population on June 30, 2001, was 19,413,200, and we've been able to project that to 20 million come Thursday.
