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Introduction

A2.1. The accuracy of ABS quarterly population estimates depends in part on the accuracy of estimates of births and deaths which are based on registrations of births and deaths.

A2.2. The major difficulty in this area stems from the fact that while the vast majority of births and deaths are registered promptly, a small proportion of registrations are delayed for months or even years. As indicated in the following tables, delays in registration are a greater problem for births than for deaths.

	A2.1: Average Lag (months) between Occurrence and Registration of Births
			Year of Registration
	State		1993	1994	1995	1996	1997
	NSW		2.1	2.1	2.1	2.0	2.4
	VIC		1.3	1.3	1.4	1.7	1.8
	QLD		1.8	1.8	1.8	1.8	1.9
	SA		1.4	1.4	1.4	1.5	1.4
	WA		1.7	1.7	1.7	1.7	1.7
	TAS		1.3	1.3	1.4	1.4	1.5
	NT		1.7	1.7	2.0	1.8	1.7
	ACT		1.6	1.6	1.7	1.4	1.5
	Australia		1.7	1.7	1.8	1.8	2.0

	A2.2: Average Lag (months) between Occurrence and Registration of Deaths
		Year of Registration
	State	1993	1994	1995	1996	1997
	NSW	1.1	1.0	1.1	1.1	1.1
	VIC	1.1	1.0	1.1	1.1	1.4
	QLD	1.1	1.2	1.1	1.1	1.0
	SA	1.0	0.9	1.0	1.0	0.9
	WA	1.0	1.0	1.0	1.0	0.9
	TAS	1.0	0.9	0.9	0.9	0.8
	NT	1.8	1.8	1.6	1.6	2.3
	ACT	1.0	1.0	1.0	1.0	1.0
	Australia	1.1	1.0	1.1	1.1	1.1

A2.3. Each quarter, preliminary estimates of birth and death occurrences are estimated directly as the number of births and deaths registered in the reference period.

A2.4. Prior to September quarter 1998, birth and death data by state of registration was converted to state of usual residence using historical patterns. This adjustment has no longer been required since September 1998 as birth and death data by state of usual residence has been directly referenced.

Revised estimates of births and deaths

A2.5. Revised estimates of quarterly birth and death occurrences are estimated by applying an inflation factor to births and death registration data.

Registration lags

A2.6. There are two types of registration delays, or lags, that we can measure for births and deaths in a given period. The historical lag, is the delay between births registered in the relevant period and their date of occurrence (ie. looks into the past such as in tables 1 and 2 ). Conversely, a future lag is the delay between births occurring in the target period and their (eventual) date of registration (ie. it looks into the future).

A2.7. The main principle behind the method used for revising birth and death occurrence estimates is that the historical and future registration lags for a quarter are similar, although this can change if registration procedures change. In other words, the historical lag derived from registration data can be used to indicate the future lag for occurrence data.

Revised estimates: estimating occurrences not yet registered

A2.8. To determine the revised estimate for a particular period, the occurrence data that will be subject to inflation needs to be determined. To enable publication by 15 months after the financial year involved, registration data for the 9 months after the end of the financial year are available. For example, to publish in September 1998, only registrations up to and including the March quarter 1998 were available for births and deaths which occurred in the 1996-97 financial year. This corresponds to 7 quarters (June Quarter 1996 to March Quarter 1998) in which births occurring in the 1996-97 financial year could have been registered. Thus, when inflating September Quarter 1996 data, occurrence data with a future registration lag of less than or equal to 7 quarters are available.

A2.9. The future lag of occurrence data available determines the length of the historical lag period used for successive calculations. To inflate September Quarter 1996 occurrences, historical lags of less than or equal to 7 quarters would be used.

A2.10. Occurrence data for a particular quarter in the year (ie. September, December, March or June) are adjusted based on registration data for the same corresponding quarter in the most recent year. This is so the most recent registration lag patterns available can be used. In the case of September Quarter 1996 occurrences, a historical lag of 7 quarters is applied to registrations in the September Quarter 1997 to obtain an inflation factor.

A2.11. The proportion, p, of all registrations for the quarter that occurred within the historical lag period is determined by:

p

=

Number of events registered within historical lag period          Total number of events registered

f

=

1    p

fS96

=

Estimate of births/deaths

=

f {Births/deaths occurring in the quarter and registered within the future lag period}

BS96

= f

{Number of births which occurred in September Qtr 1996 registered in the period September Qtr 1996 to March Qtr 1998}