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2.4. The following two sections discuss how census date estimates and the subsequent post-censal estimates are derived.
Census date population estimates
2.5. Census date population estimates are calculated for each State by sex and single years of age (from 0 years to 102 and over).
2.7. Census counts of residents are compiled for each State by single years of age and sex. ('Counts of residents' necessarily means that people counted in the census who usually reside overseas are excluded, such as visiting tourists). The question in the 1996 Census from which these data were derived was as follows:
2.8. The census counts of residents are then adjusted upwards to compensate for census net undercount. In 1996, about 1.8 per cent of the population were not counted by the census, while 0.2% were counted more than once. The precise degree of adjustment is based on estimates of undercount from the Post-Enumeration Survey and to a lesser degree comparisons between census results and independent 'demographic' and other estimates of the population (see Appendix 1: Demographic Estimates of Census Under-counting).
2.12. Estimates of the number of Australian residents temporarily overseas on census night are obtained from passenger card statistics for those Australian residents returning in the twelve month period subsequent to the census date who indicated that they were overseas on census night. These are added to the adjusted census counts derived in Step 2 to give census date population estimates. The population estimates are then converted to financial year of birth, which for the 6th August 1996 Census-date required adjusting all cohorts for the 37 day period back to 30 June by taking 37/365 of each cohort and adding it to the next one.
2.13. If the census does not fall on 30 June, a further adjustment is necessary to arrive at estimates for that date. For example, the 1996 Census was held on 6 August, and after Steps 1 to 3 were finalised, the population estimates at 6 August had to be back-dated to 30 June. This was accomplished using data from births and deaths registrations, overseas arrivals and departures data and estimates of interstate migration for the period 1 July to 6 August.
Post-censal population estimates
Estimates of total population
2.14. Using the census year (ie. 30 June) population estimates as the initial base population, post-censal estimates at the national level are compiled in accordance with births, deaths, and overseas migration. At the State level, an additional item, interstate migration, is included.
2.16. The resulting updated population estimates (ie. ) then become the base populations in the calculation of estimates for the following period. Estimation of the separate components of population change is detailed in the appendices: (Appendix 2 - Estimating Births and Deaths; Appendix 3 - Estimating Overseas Migration; and Appendix 4 - Estimating Interstate Migration).
Estimates by age
2.17. This basic procedure, outlined above, is used not only to update the total population, but also to update the age disaggregation of the population. Population estimates by single years of age are compiled quarterly by updating the census date estimates by age. These annual updates require data for the components of population change to be classified by financial year of birth in order to match each event to its corresponding cohort in the population. (Population estimates by single years of age at 30 June in effect are a classification of the population according to financial year of birth.) However, classification by financial year of birth is only available for two of the three components of change ie. births and migration. It is not available for deaths because, apart from a few exceptions (each State has its own notification form), death certificates only require notice of the deceased person's age at the time of death; they do not require date of birth. Knowledge of a person's age is not sufficient information to determine whether that person's birthday has already occurred in the current financial year.
2.20. For deaths aged 0, the separation factor calculated from Australian data is around 0.85, indicating that of the deaths aged 0 in a particular year, 85 per cent were born in that year and 15 per cent were born in the previous year.
2.21. For deaths aged 1, the separation factor is around 0.60. For deaths aged 2 and over, the separation factor is about 0.50, indicating an even distribution of deaths over time at those ages.