HOW WE PRODUCE THE PROJECTIONS
Babies
Two factors will determine how many babies are likely to be born in the future: the number of women who are of the right age to have children, and how many babies they may have.
To estimate how many babies are likely to be born in NSW in the years to come, we analyse births statistics from the ABS and the ACT, Queensland, Victoria and NSW Ministry of Health’s respective Perinatal Data Collections to understand what NSW birth rates have done in previous years.
We first calculate ‘age-specific fertility rates’ (ASFRs), or the number of babies born to women aged between 15 and 49 years divided into five-year age groups. These rates are then used to calculate the total fertility rate (TFR), or the number of babies likely to be born to a woman over her lifetime. This process is undertaken at the NSW, region and LGA geography levels.
Deaths
We need to know the age at which people are likely to die, or ‘life expectancy’, in order to estimate the size of our future population. This is the mortality assumption used in the projections.
We use the latest ABS data on the number of deaths by age, together with ERP data to derive a current ‘age-specific death rate’. This is used to update a master life table, which derives a probability of death at each age. Through iterative calculations, the life table calculates a life expectancy at each five-year age group for women and men. This is the average age at which a newborn can expect to live, and this differs between women and men. The master life table builds on historic deaths data back to 1975 and uses this to forecast future life expectancies. Once the life expectancy at birth figure is derived, we then compare our assumptions with figures produced by the ABS and the United Nations to make sure they are reasonable.
Migration
Understanding where and when people move home is a key part of creating our population projections. Whether moving to the other side of the country, or a few suburbs away, all moves change the local population profile. Without population mobility we would be assuming that everyone lives in the same home from birth to death, which is very rare.
We consider it in three different ways for our population projections:
Intrastate movements: People moving within NSW, which is the most frequently-occurring type of move.
Interstate movements: People moving between NSW and other states or territories in Australia. For interstate and intrastate moves, we used 2016 Census data to calculate the likelihood of someone moving based on their age, where they live, and where they are likely to move to.
Overseas migration: People moving into or out of Australia on a long-term or permanent basis. We use short-term forecasts from the Department of Home Affairs as the starting point for the overseas migration assumptions. The forecasts are based on the latest data on visa grants, past overseas migration flows by visa group, existing migration policy decisions and official economic outlooks.
The places we anticipate people moving to in Sydney are also influenced by future housing development and current government policies.