Assessing mortality from road injury


A commonly used public health measure is the number of deaths per 100,000 population, but to accurately evaluate the impact of road injury the measure should take into account the extent of exposure to death from a road injury. Possible measures include the numbers of death from road injury per hours or distance travelled or per 10,000 motor vehicles, but neither of these measures takes account of the number of passengers per vehicle. Also, neither of these measures takes account of pedestrian or bicycle activity. Because of the difficulties and limitations with these measures, the assessment of mortality from road injury generally uses the standard public health measure of deaths per 100,000 population.

The rate of deaths per 100,000 population, which is known as crude death rate, is the measure used most commonly to express the overall impact of road injury (at national and state levels, for example). As the name implies, however, a crude death rate is a limited indicator of deaths, as it doesn’t take account of the age and sex structure of populations. As such, it does not reflect variations in the numbers of deaths between the sexes and at various ages, and is not a suitable measure for comparing populations with different age and sex structures. This is particularly important in considering Indigenous mortality, because of the substantial differences in the age structures of the Indigenous and non-Indigenous populations.

A detailed comparison of death rates involves the use of age-sex-specific rates. These rates illustrate the variations in death rates by age and sex. An age-specific death rate is the number of deaths of persons of a specific age group in one year per 100,000 persons of the same age/sex group. Conventionally, five-year age groups are used with the first five-year group (0-4 years) sometimes broken down into 0 and 1-4 year sub-groups. Deaths at very old ages are generally grouped into 85 years and over. The problem with age-sex-specific rates is that it is difficult to compare populations (because of the many comparisons involved) - often a single summary measure of mortality is required.

Standardisation is a process for adjusting for the differences in age structures of populations, both between different populations and between the same population at different times. An age-standardised rate (also referred to as an age-adjusted rate) is a summary measure of the death rate that a population would have if it had a standard age structure. Calculation of the age-standardised rate involves summing the products of the population's age-specific rates and the standard population for each age group. This method is generally termed direct standardisation.

An alternative approach, generally termed indirect standardisation, involves summing the products of a standard set of rates and the population in each age group of the population being considered. In this way, it is possible to estimate the number of deaths expected in the population being considered if it experienced the age-specific death rates of the reference population. The ratio of the number of deaths observed to the number expected is known as the standardised mortality ratio (SMR). If the ratio is greater than one, there were more deaths than would have been expected. If it is less than one, there were fewer deaths than would have been expected. An SMR of 2.5, for example, would indicate that there were 2.5 times as many deaths as expected.

SMRs are used in this summary to compare the impact of road deaths on the Indigenous and total populations.
Comparison of the impact of injury resulting in hospitalisation uses age-standardised rates for the Indigenous and total populations.

  • Australian Bureau of Statistics, Australian Institute of Health and Welfare (2003) The health and welfare of Australia's Aboriginal and Torres Strait Islander peoples 2003. (ABS catalogue no. 4704.0) Canberra: Australian Institute of Health and Welfare and the Australian Bureau of Statistics
  • Australian Transport Safety Bureau (2004) Road safety in Australia: a publication commemorating World Health Day 2004. Canberra: Australian Transport Safety Bureau
  • Last JM, ed. (2001) A dictionary of epidemiology. 4th ed. New York: Oxford University Press
  • Rothman K, & Greenland S (1998) Modern epidemiology. Second ed. Philadelphia, PA: Lippincott-Raven Publishers.

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Last updated: 19 June 2006