FEATURE ARTICLE: FINAL REBASING AND REVISION OF AUSTRALIA'S POPULATION ESTIMATES, SEPTEMBER QUARTER 2001 – JUNE QUARTER 2006


REBASING

After each Census, the Australian Bureau of Statistics (ABS) uses the new information obtained to rebase the estimated resident population (ERP) of Australia and its States and Territories.

The 2006 Census of Population and Housing (2006 Census) has been used to construct a new base population figure for 30 June 2006. Because this new population estimate uses the Census as its main data source, it is said to be 'based' on that Census and is referred to as a population base for population estimates and projection, for subsequent reference periods. Rebasing refers to the process by which the ABS uses this new base derived from the 2006 Census to update the original series of published quarterly population estimates from 30 September 2001 to 30 June 2006 (the previous intercensal period). These original series of estimates (based on the 2001 Census) are now superseded by new more accurate estimates based on the 2006 Census.

Preliminary Rebasing

In Australian Demographic Statistics, December Quarter 2006 (cat. no. 3101.0), the ABS used the 2006 Census results to produce 'preliminary rebased' population estimates for all previous intercensal quarters, including a 'preliminary base' ERP for 30 June 2006.

Component Revision

In Australian Demographic Statistics, March Quarter 2007 (cat. no. 3101.0), these population estimates were updated due to revisions of the components of intercensal population change. The ERP as at 30 June 2006, however, remained as the 'preliminary base' and did not change as a result of this process.

Final Rebasing

In this release, the population estimates have again been updated to produce the 'final rebased' estimates, including a new 'final base' ERP for 30 June 2006. Revisions have also been made to the intercensal birth data. After these updates, no subsequent revisions will be made to these estimates, although post censal quarters, from the September Quarter 2006, will be revised as part of the scheduled annual revision cycle.

The remainder of this article provides a more detailed explanation of the adjustments made using the 2006 Census results and some of the related rebasing concepts.

FINAL REBASING

Constructing the final ERP figure for Census night

To construct the final rebased ERP for Census night, two adjustments have been made from the preliminary rebasing. This involved:

• a revision to the number of Australian residents who were temporarily overseas (RTOs) on Census night using data on international travellers obtained from the Department of Immigration and Citizenship. This measure could only be obtained 21 months after Census night, when actual traveller behaviour, and each traveller's true residence status on Census night (according to the 12/16 month rule) could be observed. The availability of final data has resulted in a small revision. Final RTOs numbered 346,000 compared with preliminary RTOs numbering 345,200;
• applying a range of demographic adjustments designed to resolve any statistical anomalies in the age sex composition of the derived population estimates.

Backdating from Census night to 30 June 2006

A key step in arriving at the 'final base' ERP figure for 30 June 2006, was to backdate the resulting ERP figure for Census night (8 August 2006) to 30 June 2006 using revised components of population change. This involved:

• subtracting births;
• adding deaths; and
• subtracting net interstate migration and net overseas migration.

Compared to the preliminary rebasing, the revisions due to backdating from Census night to 30 June 2006 in the final rebasing are small.

Adjustment Components

Table 1 shows figures that have been produced, following the final rebasing process, which include updating RTOs and demographic adjustments.

These calculations were undertaken to ensure that population estimates for the next intercensal period (i.e. 2006-2011) are as accurate as possible.

Table 1, Adjustment components of estimate resident population, final - 30 June 2006
		NSW	Vic	Qld	SA	WA	Tas	NT	ACT	Aust(a)
		'000	'000	'000	'000	'000	'000	'000	'000	'000
PERSONS
Components as at 8 August 2006:
	Census count, actual location	6 585.7	4 915.3	4 046.9	1 509.0	1 986.2	470.8	217.1	327.9	20 061.6
	plus - Residents absent interstate	74.8	87.7	38.1	28.4	19.2	13.0	4.5	10.5	276.3
	less - Interstate visitors	54.3	36.2	107.5	13.8	25.1	5.5	22.1	11.4	276.3
	less - Overseas visitors	57.0	34.4	72.9	9.2	21.3	1.8	6.7	2.9	206.4
equals - Census count, place of usual residence		6 549.2	4 932.4	3 904.5	1 514.3	1 959.1	476.5	192.9	324.0	19 855.3
	plus - Allowance for under-enumeration	157.6	113.6	148.4	36.3	64.2	9.5	15.9	4.0	549.6
	plus - Demographic adjustment	-5.1	-3.4	-2.8	-1.0	-1.3	-0.3	-0.2	-0.2	-14.3
	plus - Residents Temporarily Overseas	124.3	94.6	50.5	20.2	42.5	4.6	2.5	6.8	346.0
equals - ERP as at 8 August 2006		6 826.0	5 137.3	4 100.6	1 569.8	2 064.4	490.3	211.1	334.7	20 736.6
Backdating components to 30 June 2006:
	less - Births	9.6	7.1	6.0	2.0	2.9	0.7	0.4	0.5	29.3
	plus - Deaths	5.4	3.9	2.9	1.4	1.3	0.4	0.1	0.2	15.6
	less - Net Interstate Migration	-2.4	-0.1	2.4	-0.2	0.4	-0.1	0.1	-	-
	less - Net Overseas Migration	8.2	7.6	4.2	1.5	3.1	0.1	0.1	0.2	25.0
equals - final ERP 30 June 2006		6 816.1	5 126.5	4 090.9	1 567.9	2 059.4	490.0	210.6	334.1	20 697.9
- nil or rounded to zero (including null cells)
(a) Includes Other Territories - see paragraph 2 of the Explanatory Notes.

REVIEW OF INTERCENSAL COMPONENTS OF GROWTH

Revisions to intercensal births for 2001-2006

The annual birth and death revision cycle as part of ERP compilation is designed to estimate all vital events occurring in a reference financial year, including lagged events - that is births and deaths which have occurred but not yet been registered with the relevant state and territory Registrar of Births, Deaths and Marriages. The final revision to ERP during rebasing allowed another opportunity to revise the births estimates during the 5 year intercensal period. For this rebasing, only births were revised and not deaths, as the alternative death figures did not change significantly enough to warrant a revision. This does not impact on ABS' core fertility or mortality rates as these used year of registration data, which does not undergo an estimation/revision process.

Table 2, Births by financial year of occurrence - final(a)
	New South Wales	Victoria	Queensland	South Australia	Western Australia	Tasmania	Northern Territory	Australian Capital Territory	Australia(b)
Period	no.	no.	no.	no.	no.	no.	no.	no.	no.
2001-02	83 157	60 401	48 157	17 600	24 390	5 831	3 740	3 978	247 288
2002-03	83 597	60 220	47 896	17 330	24 025	5 723	3 818	4 023	246 663
2003-04	82 877	61 440	49 514	17 339	24 419	5 707	3 620	4 142	249 082
2004-05	84 582	62 306	52 217	17 655	25 453	5 978	3 514	4 202	255 934
2005-06	86 177	63 815	54 089	17 999	26 930	6 372	3 726	4 409	263 540
(a) Final estimates published in the next issue of Australian Demographic Statistics (cat. no. 3101.0) to be released on 24 June 2008.
(b) Includes Other Territories - see paragraph 2 of the Explanatory Notes.

Table 3, Births by financial year of occurrence - revised estimates(a)
	New South Wales	Victoria	Queensland	South Australia	Western Australia	Tasmania	Northern Territory	Australian Capital Territory	Australia(b)
Period	no.	no.	no.	no.	no.	no.	no.	no.	no.
2001-02	84 085	60 507	47 652	17 579	23 967	5 871	3 739	3 959	247 436
2002-03	84 893	60 467	47 317	17 286	23 791	5 758	3 815	4 014	247 408
2003-04	85 714	61 907	49 189	17 249	24 530	5 734	3 615	4 160	252 123
2004-05	84 233	62 658	52 092	17 608	25 439	6 047	3 517	4 226	255 846
2005-06	86 000	63 804	54 199	17 987	26 908	6 371	3 724	4 414	263 430
(a) Estimates previously published in Australian Demographic Statistics, September Quarter 2007 (cat. no. 3101.0).
(b) Includes Other Territories - see paragraph 2 of the Explanatory Notes.

Table 4, Differences(a)
	New South Wales	Victoria	Queensland	South Australia	Western Australia	Tasmania	Northern Territory	Australian Capital Territory	Australia(b)
Period	no.	no.	no.	no.	no.	no.	no.	no.	no.
2001-02	928	106	-505	-21	-423	40	-1	-19	148
2002-03	1 296	247	-579	-44	-234	35	-3	-9	745
2003-04	2 837	467	-325	-90	111	27	-5	18	3 041
2004-05	-349	352	-125	-47	-14	69	3	24	-88
2005-06	-177	-11	110	-12	-22	-1	-2	5	-110
(a) Positive differences mean registered births in Table 2 are less than the estimates in Table 3; negative differences mean registered births in Table 2 are greater than the estimates in Table 3.
(b) Includes Other Territories - see paragraph 2 of the Explanatory Notes.

Table 5, Percentage differences(a)
	New South Wales	Victoria	Queensland	South Australia	Western Australia	Tasmania	Northern Territory	Australian Capital Territory	Australia(b)
Period	%	%	%	%	%	%	%	%	%
2001-02	1.1	0.2	-1.1	-0.1	-1.8	0.7	-	-0.5	0.1
2002-03	1.5	0.4	-1.2	-0.3	-1.0	0.6	-0.1	-0.2	0.3
2003-04	3.3	0.8	-0.7	-0.5	0.5	0.5	-0.1	0.4	1.2
2004-05	-0.4	0.6	-0.2	-0.3	-0.1	1.1	0.1	0.6	-
2005-06	-0.2	-	0.2	-0.1	-0.1	-	-0.1	0.1	-
- nil or rounded to zero (including null cells)
(a) Positive differences mean registered births in Table 2 are less than the estimates in Table 3; negative differences mean registered births in Table 2 are greater than the estimates in Table 3.
(b) Includes Other Territories - see paragraph 2 of the Explanatory Notes.

These birth tables compare the revised estimates of births which were released 15 months after each reference period (shown in table 3) with the final estimates released in this issue. Table 2 shows the final estimates for the 2001-2006 intercensal period.

Table 4 shows that the 2003-04 financial year was subject to the highest level of revision with the previous births figures being overstated by 3,000 across Australia. Of this, New South Wales was the highest contributor with an overstatement of 2,800 births. Over the intercensal period, the Queensland births were understated for all years other than 2005-06, while Victorian births were overstated for all years other than 2005-06.

Table 5 shows that the two Territories and South Australia experienced negligible change in their births estimates as part of this final revision process. Table 5 also shows that this final revision process resulted in relatively small changes to the number of births.


UPDATING THE INTERCENSAL ERP FIGURES DURING THE 'FINAL REBASING' CYCLE

After the first series of calculations were applied to construct a final ERP base figure for Australia and each of the states and territories at 30 June 2006, a second series of calculations was made. These calculations updated the nineteen intercensal quarters preceding June quarter 2006 (i.e. September quarter 2001 - March quarter 2006) to produce 'final rebased' population estimates.

In the case of the five-year intercensal period (i.e. 19 quarters), the population at the end of the first quarter is adjusted up or down by 1/20 of the intercensal discrepancy, the second quarter is adjusted by 2/20, the third by 3/20, etc., whereas the final quarter (the new base figure) is not adjusted. Details of the intercensal discrepancy for the 2001-06 intercensal period are available further on in this article.

The main reasons for updating these 19 quarterly intercensal ERP figures is to provide a more accurate population estimate for each quarter and also to ensure that the estimates from the 2001-2006 intercensal period will be comparable with all future estimates, thus creating a consistent time series of ERP data.

These adjustments complete the final rebasing process and following this, no subsequent revisions will be made to these estimates.


Intercensal Error and Intercensal Discrepancy

The intercensal error refers to the difference between the latest 2006 Census based ERP figures for 30 June 2006 and the ERP figures based on the 2001 Census which have been carried forward using births, deaths and migration data.

There are two components that contribute to the intercensal error:

• errors in the Census based estimates of the population at the 2001 or 2006 Census date; and/or
• errors in the estimates of any of the components of population change (births, deaths and migration) during the intercensal period.

During the rebasing process some components of population change are updated as more complete data becomes available. Therefore, 'intercensal error' can be revised a few times during the rebasing process, but it is finalised with the final rebasing.

The 2006 Census has allowed the ABS to estimate approximately how much of the intercensal error is due to inaccuracies in modelled estimates of interstate migration. In order to do this, the ABS has analysed data from the Census questions concerning an individual's place of usual residence one year ago, and five years ago and at Census night.

After the intercensal error is adjusted for revisions using Census data on interstate migration, the remaining unattributable portion is referred to as the intercensal discrepancy. The estimate of intercensal discrepancy for each state and territory, birth cohort and sex are spread evenly across the intercensal quarters. Thus the intercensal discrepancy acts as a balancing item, that when combined with births, deaths and migration equals the difference between the two 30 June Census year population estimates.

The table below shows the changes from 'preliminary intercensal error' to 'updated intercensal error', 'preliminary intercensal discrepancy' and 'final intercensal error' and 'final intercensal discrepancy' by number and percentage of total population for the 2001-2006 period.

Table 6, Intercensal error and discrepancy 2001 - 2006(a)
	PRELIMINARY REBASING CYCLE		COMPONENT REVISION CYCLE				FINAL REBASING CYCLE
	Preliminary Intercensal Error		Updated Intercensal Error		Preliminary Intercensal Discrepancy		Final Intercensal Error		Final Intercensal Discrepancy
	'000	%	'000	%	'000	%	'000	%	'000	%
New South Wales	10.5	0.15	6.2	0.09	2.0	0.03	2.8	0.04	-1.5	-0.02
Victoria	-36.6	-0.71	-36.8	-0.72	-39.7	-0.77	-36.2	-0.71	-39.1	-0.76
Queensland	-38.1	-0.93	-26.1	-0.64	-29.9	-0.73	-24.0	-0.59	-27.9	-0.68
South Australia	-13.5	-0.86	-13.3	-0.85	-12.0	-0.77	-12.8	-0.82	-11.5	-0.73
Western Australia	-8.2	-0.40	-7.5	-0.36	-3.4	-0.17	-7.3	-0.35	-3.1	-0.15
Tasmania	-1.0	-0.20	-0.6	-0.12	-0.6	-0.11	-0.8	-0.16	-0.6	-0.12
Northern Territory	-4.0	-1.89	-3.0	-1.43	-0.7	-0.31	-2.9	-1.40	-0.7	-0.32
Australian Capital Territory	-5.4	-1.62	-4.8	-1.42	-1.5	-0.44	-4.7	-1.40	-1.4	-0.42
Australia(b)	-96.0	-0.46	-85.6	-0.41	-85.6	-0.41	-85.7	-0.41	-85.7	-0.41
(a) A positive number indicates that unrebased ERP as at 30 June 2006 was higher than the rebased ERP. A negative number indicates it was lower than the rebased ERP.
(b) Includes Other Territories - see paragraph 2 of the Explanatory Notes.

Table 7 shows 'final intercensal discrepancy' by five year age groups.

The age group with the highest level of discrepancy was the 0-4 year age group with a discrepancy of minus 39,500. This was followed by the 20-24 year age group with a discrepancy of minus 26,200. The smallest discrepancy occurred in the 60-64 year age group with a figure of minus 600.

Although 'final intercensal discrepancy' is reported as a quality indicator on the unrebased 30 June census year population estimate, it is considered to have accumulated over the entire intercensal period.

Table 7, Final intercensal discrepancy by five year age groups(a)(b)
	FINAL INTERCENSAL DISCREPANCY
	Persons	Persons
	'000	%
0-4	-39.5	-3.02
5-9	-19.4	-1.45
10-14	-8.0	-0.57
15-19	-17.9	-1.26
20-24	-26.2	-1.78
25-29	-5.0	-0.36
30-34	-4.9	-0.33
35-39	-6.5	-0.43
40-44	-5.0	-0.32
45-49	-0.3	-0.02
50-54	-8.0	-0.58
55-59	15.1	1.19
60-64	-0.6	-0.06
65-69	11.0	1.41
70-74	5.2	0.83
75-79	6.9	1.24
80-84	5.1	1.25
85 and over	12.2	3.79
All age groups	-85.7	-0.41
(a) A positive number indicates that unrebased ERP as at 30 June 2006 was higher than rebased ERP. A negative number indicates it was lower than rebased ERP.
(b) Includes Other Territories - see paragraph 2 of the Explanatory Notes.