3114.0 - Demography Working Paper 1998/2 - Quarterly Birth and Death Estimates, 1998
ARCHIVED ISSUE Released at 11:30 AM (CANBERRA TIME) 09/03/1999
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Demography Working Paper 98/2 Abstract The estimated number of births and deaths are major components of ABS quarterly State and Territory population estimates. A small proportion of these events are not registered for months, or even years, after they have occurred. As a result, ABS quarterly estimates have slightly underestimated the true numbers of births and deaths. This paper describes a method by which an inflation factor can be applied to births and deaths data to estimate the true number of such events occurring in a particular period. This paper is a slightly revised version of one presented at the 1998 Australia-New Zealand Population Workshop, 18-20 November 1998. Introduction 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. 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.
Following an approach from the Queensland Government Statistician's Office, the ABS proposes to estimate the number of births and deaths occurring in a particular quarter for population estimates by applying an inflation factor to births and death registration data. Investigations outlined below appear to support introduction of this method for revised population estimates. Preliminary, revised and final estimates There are three stages in the production of birth and death estimates for population estimate purposes. Preliminary estimates of births and deaths are produced 6 months after the reference date using period of registration data as a proxy for period of occurrence data. Preliminary estimates for a financial year are revised 15 months after the year using period of occurrence data. The revised estimates are then held constant until they are finalised after the next Census of Population and Housing using a later period of occurrence data. Prior to 1991, final period of registration data was used for both the revised and final estimates. The major problems inherent in this process are that preliminary estimates are inconsistent with final estimates, revised estimates exclude births and deaths registered more than 15 months after the financial year and final estimates do not reflect births or deaths that have not been registered at the time finalisation occurs. Current ABS revised and final estimates slightly undercount the true numbers of births and deaths occurring in a particular period. Registration lags There are two types of registration delays, or lags, that we can measure for births and deaths in a given period. The first, referred to in this paper as historical lag, is the delay between births registered in the relevant period and their date of occurrence (ie it looks back into history 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). The main principle behind ABS' proposed method 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 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 year involved, occurrence data available for up to 14 months after a financial year can be used. For example, ABS had data for births and deaths which occurred in the 1996-97 financial year and were registered up to and including March Quarter 1998 by August 1998 (14 months after 1996-97). 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 is available. 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. Under ABS' proposed inflation method, occurrence data for a particular quarter type (September, December, March or June) are adjusted based on registration data for the latest quarter of the same type. In the case of September Quarter 1996 occurrences, a historical lag of 7 quarters is applied to registrations for September Quarter 1997. The proportion, p, of all registrations for the quarter that occurred within the historical lag period is determined by: The inflation factor, f, for the quarter can be calculated such that: For example, the inflation factor for births occurring in September Quarter 1996 would be: The estimate is calculated by applying the inflation factor to the count of births/deaths on file, such that: Other inflation methods Two other methods of generating inflation factors were also investigated by the ABS. The only difference between the three inflation methods is in the registration data used to generate historical lags. The first alternative uses the historical registration lag of a particular quarter to determine the future registration lag for that same quarter. The inflation factor generated using this method for the September Quarter 1996 is therefore: The second alternative method uses the most up-to-date data for both registrations and occurrences to estimate historical and future registration lags. To achieve this, it looks at lags over an extended period rather than lags for a particular quarter. The inflation factor generated using this method for the September Quarter 1996 is: In concept, the proposed method has the advantage of use of more recent data than the first alternative and reflects seasonality better than the second alternative. Results Quarterly revised estimates of births and deaths from the June Quarter 1985 to the June Quarter 1996 were created using ABS' proposed inflation method and counts of occurrences. These estimates were then compared to the occurrence counts available for each quarter, updated for registrations up to March 1997. Annual results appear in Attachment 1. A positive difference between a revised estimate and the latest count on file means that more registrations for the period involved are expected. A negative difference means that more registrations for the period involved have been received than expected. Since there is less time for late registrations to take place, differences in recent years should be predominantly positive. The results indicate only minor differences existing between actual occurrences of births and deaths (latest count on file) and revised estimates, particularly for early years. Comparison with current method results for preliminary, revised and final estimates in 1991-92 are shown in Table 3.
Estimates for the alternative methods were also produced and show similar results. For births there is negligible difference between the methods in the period 1985-86 to 1990-91. After 1991 the second alternative method appears to provide better results, but this reflects a probable conservative estimate by not fully taking into account occurrence that have yet to be registered. For deaths, the proposed method provides a mid-range estimate. Conclusions The proposed method would provide a better revised estimate of the true number of births and deaths than is available from registration data alone. Subject to further investigation, ABS also proposes to apply an inflation factor to preliminary and final estimates. For preliminary estimates this is likely to involve alternative method 1, while the method proposed for revised estimates is likely to be suitable for final estimates. Attachment 1. Revised estimates of birth and death occurrences by State or Territory of usual residence
Attachment 1. Revised estimates of birth and death occurrences by State or Territory of usual residence (continued)
Attachment 1. Revised estimates of birth and death occurrences by State or Territory of usual residence (continued)
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