When measuring and comparing the effect of mortality in populations, several important concepts are used:
- Number of deaths;
- Mortality rates;
- Years of life lost (YLL); and
- Life expectancy.
The number of deaths is a simple measure which readily quantifies the mortality impact on the population.
The mortality rates used in this site are age-standardised mortality rates (ASMR), allowing populations to be compared more meaningfully by eliminating the effect of differences in their age structures. They combine and summarise the age-sex specific rates using a comparative population as the standard population. In this site age-standardised rates are simply referred to as the mortality rate. These rates are standardised to the estimated resident population of Australia at 30 June 2001 and are expressed as number of deaths per hundred thousand population. Mortality rates are particularly useful when comparing deaths over time.
Years of life lost is a concept which attempts to measure the number of years of life lost per annum due to death as a result of a specific cause, given life expectancies at specific ages. Deaths before age 75 are regarded as premature for both men and women which means that a person dying at age 50 contributes 25 years to the measure of years of life lost. This measure helps to highlight those causes of death that affect younger people and is usually the major component when estimating the burden of particular diseases.
Life expectancy is the average number of years a person is expected to live given that the current mortality rates continue to prevail. It can be specified as life expectancy at birth or as years remaining at different ages.
It is important to note that mortality rates and actual numbers of deaths do not always change in the same way. For example the number of deaths may rise over time because the population has more people living into old age, but rates at particular ages and the crude rate may fall if the total population numbers increase faster than the number of deaths.