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2.46. When the census year (ie. 30 June) population estimates become available for the States and Australia they can be compared with the alternative estimates for the same date already produced by updating the previous census date estimate (using births, deaths and migration data). The difference between the two estimates for each State and Australia is called the 'intercensal error'.
2.50. The accuracy of Australia's post-censal population estimates has improved over time and compares favourably with that of other countries (see Table 2.3).
2.51. There are two areas which contribute to the intercensal error:
An assessment of the accuracy of census date population estimates and the components of population change is given in Chapter 4.
Intercensal Revision 1991-96
2.52. For the 1991-96 State intercensal estimates, the 1996 Census allowed an assessment of how much of the intercensal error was due to inaccuracies in estimates of interstate migration. The 1996 census data which allowed this assessment were from the two census questions on usual residence 1 year ago and 5 years ago. Estimates of interstate migration based on census data were used to revise the interstate migration component of population updates for the intercensal period, before the intercensal discrepancy was calculated.
(a) Revisions of interstate migration
2.55. Comparisons of the Medicare-based estimates of interstate migration (see Appendix 6) for the 1991-96 period, with 1996 census counts of 1991-96 interstate migration (adjusted for undercount and to include an estimate for ages 0 to 4), are shown in Table 2.4.
2.56. The approach used for revising population estimates for the intercensal period assumes that intercensal error is primarily due to errors in estimating interstate migration (as it is the most difficult component to estimate), to the extent that this is supported by the census-based migration data. If the difference between census-based and Medicare-based migration estimates was the same sign (positive or negative) as a State's intercensal error, then that migration difference was interpreted as accounting for the intercensal error.
2.57. For example, NSW's preliminary intercensal error of -13,738 could be entirely attributed to interstate migration being 14,803 lower in Medicare-based migration than census-based migration. As an adjustment of only 13,738 is required to explain the intercensal error, that was the amount the Medicare-based estimates were altered by (giving -64,740 in Table 2.5), rather than the full Medicare-based migration difference of 14,803.
2.58. The intercensal errors for South Australia and the Australian Capital Territory were not explained at all by the census-based migration data, so their Medicare-based migration estimates were not altered. For instance, South Australia had an intercensal error of -4,764 (ie. the 1991-based 30 June 1996 population estimate was 4,764 too high), but census-based migration indicated that, in net terms, fewer people departed the State. As such, if South Australia's migration estimate was adjusted according to census-based migration, its intercensal error would increase.
2.59. As the intercensal error sums to 26,405 rather than zero, using it to help quantify the extent of interstate migration revisions means that a further adjustment is required to ensure interstate migration sums to zero for Australia, as shown in Table 2.5. These 26,405 persons were redistributed across all States in proportion to each State's share of total interstate migration flows, according to 1991-96 Medicare-based interstate arrivals and departures.
2.60. The preliminary intercensal discrepancy is the preliminary intercensal error adjusted for the revisions made to interstate migration (see Table 2.6).
2.61. The final intercensal discrepancy is the preliminary intercensal discrepancy adjusted for finalised estimates of births, deaths and category jumping which occurred between 1991 and 1996. As there is a lag before finalisation of these components of population change, final intercensal discrepancy is calculated considerably later than the preliminary levels.
2.62. The final intercensal error, as shown in Table 2.2, is equivalent to the final intercensal discrepancy but without the interstate migration revisions introduced to account for intercensal error. They are therefore identical at the Australian, but not State level.
2.63. The 1991-96 revised interstate migration estimates were then divided into two parts:
2.64. The 1995-96 revised net interstate migration levels [2.63(i)] were derived by:
Calculating the difference between the original 1991-96 Medicare-based estimates and the final revised figures.
Expressing the result of Step 1 as a proportion of the difference between the original 1991-96 Medicare-based estimates and the 1991-96 census migration estimates.
Applying the proportions from Step 2 to the gap between the original 1995-96 Medicare-based estimates and the 1995-96 census migration estimates.
Adjusting the revised net interstate migration levels in Step 3 to ensure they sum to zero.
2.65. Quarterly interstate arrival and departure estimates for 1995-96 and for the four years 1991-95 were calculated by spreading the total net gain/loss for each State over the 4 quarters for 1995-96 and the 16 quarters for 1991-95 proportionally according to the original quarterly Medicare-based estimates.
(b) Distribution of the intercensal discrepancy
2.66. The 1991-96 intercensal discrepancy was distributed evenly over the intercensal period for each State, sex and birth cohort as follows:
2.69. These formulae were also used for distributing the intercensal discrepancy over the quarterly population estimates by marital status and country of birth.
(c) Final revised intercensal population estimates
2.70. The revised estimates of the population for the intercensal period were calculated by the quarterly adjustment of the population, starting with the 30 June 1991 (Census-based) population, for subsequent births, deaths, overseas migration, interstate migration and intercensal discrepancy. This was the same procedure as outlined in paragraph 2.15 for post-censal estimation, except that finalised components of change were available and the intercensal discrepancy component was also included.