## Methods and conventions

### Age-standardisation

This is a method of removing the influence of age when comparing populations with different age structures—either different populations at the same time or the same population at different times. For this report, the Australian estimated resident population as at 30 June 2001 has been used as the standard population. The same population was used for males and females to allow valid comparison of age-standardised rates between the sexes (see Table A1).

Two different methods of age-standardisation can be used: direct and indirect. Direct age-standardisation has been used in this report.

### Direct age-standardisation

This is the most common method of age-standardisation, and is used in this report for prevalence, hospitalisations and most deaths data. This method is generally used when the populations under study are large and the age-specific rates are reliable. The calculation of direct age-standardised rates has 3 steps:

Step 1: Calculate the age-specific rate for each age group.

Step 2: Calculate the expected number of cases in each age group by multiplying the age-specific rate by the corresponding standard population for each age group.

Step 3: Sum the expected number of cases in each age group and divide this sum by the total of the standard population to give the age-standardised rate.

Table A1: Age composition of the Australian population at 30 June 2001
Age group (years) Australia, 30 June 2001
0 253,031
1-4 1,029,326
5-9 1,351,664
10-14 1,353,177
15-19 1,352,745
20-24 1,302,412
25-29 1,407,081
30-34 1,466,615
35-39 1,492,204
40-44 1,479,257
45-49 1,358,594
50-54 1,300,777
55-59 1,008,799
60-64 822,024
65-69 682,513
70-74 638,380
75-79 519,356
80-84 330,050
85 and over 265,235
Total 19,413,240

Source: ABS 2003.

### Average annual rates of change

Average annual rates of change or growth rates have been calculated as geometric rates:

where

Pn = value in later time period

Po = value in earlier time period

N = number of years between the two time periods.

### Classification of diseases

The classification of deaths follows the 10th revision of the International Classification of Diseases (WHO 1992). Diseases treated in hospitals are classified using the International statistical classification of diseases and related health problems, 10th revision, Australian Modification (ICD-10-AM). The procedures performed during a hospital stay are classified using the Australian Classification of Health Interventions.

### Data subject to revision

This report draws data from a range of administrative and survey data sets, all of which are subject to change. Such changes may arise from the:

• nature of the data collection—for example, notifiable diseases are based on a 'real-time' collection of reported cases, and the counts and rates could change daily
• data being updated on a regular cycle—for example, childhood immunisation rates are calculated each quarter
• data being revised as part of a program of maintenance and updates—for example, for 2007 data onwards the Australian Bureau of Statistics (ABS) revises cause-of-death data using information pertaining to coroners' cases that was not available at the time of original processing
• data being revised because of discovered errors or anomalies.

Wherever possible, the latest version of a data set has been used; in cases where the data change frequently, the date of the release is noted in the text or table.

Note: Cause of Death Unit Record File data are provided to the AIHW by the Registries of Births, Deaths and Marriages and the National Coronial Information System (managed by the Victorian Department of Justice) and include cause of death coded by the ABS. The data are maintained by the AIHW in the National Mortality Database.

### References

ABS (Australian Bureau of Statistics) 2003. Australian demographic statistics, September quarter 2002. ABS cat. no. 31010.0. Canberra: ABS.

WHO (World Health Organization) 1992. International Statistical Classification of Diseases and Related Health Problems. 10th revision, volumes 1 and 2. Geneva: WHO.