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epidemiology Epidemiology is the study and analysis of the distribution (who, when, and where), patterns and determinants of health and disease conditions in a defined population. It is a cornerstone of public health, and shapes policy decisions and evidenc ...
and
demography Demography () is the statistics, statistical study of populations, especially human beings. Demographic analysis examines and measures the dimensions and Population dynamics, dynamics of populations; it can cover whole societies or groups ...
, age adjustment, also called age standardization, is a technique used to allow
statistical population In statistics, a population is a set of similar items or events which is of interest for some question or experiment. A statistical population can be a group of existing objects (e.g. the set of all stars within the Milky Way galaxy) or a hypoth ...
s to be compared when the age profiles of the populations are quite different.


Example

For example, in 2004/5, two Australian health surveys investigated rates of long-term
circulatory system The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...
health problems (e.g.
heart disease Cardiovascular disease (CVD) is a class of diseases that involve the heart or blood vessels. CVD includes coronary artery diseases (CAD) such as angina and myocardial infarction (commonly known as a heart attack). Other CVDs include stroke, hea ...
) in the general Australian population, and specifically in the
Indigenous Australian Indigenous Australians or Australian First Nations are people with familial heritage from, and membership in, the ethnic groups that lived in Australia before British colonisation. They consist of two distinct groups: the Aboriginal peoples ...
population. In each age category over age 24, Indigenous Australians had markedly higher rates of circulatory disease than the general population: 5% vs 2% in age group 25–34, 12% vs 4% in age group 35–44, 22% vs 14% in age group 45–54, and 42% vs 33% in age group 55+. However, overall, these surveys estimated that 12% of all Indigenous Australians had long-term circulatory problems compared to 18% of the overall Australian population. To understand this "apparent contradiction", we note that this only includes age groups over 24 and ignores those under. Indigenous figures are dominated by the younger age groups, which have lower rates of circulatory disease; this masks the fact that their risk at each age is higher than for non-Indigenous peers of the same age, if you simply pretend that no Indigenous people are under 24.


Weighting

To get a more informative comparison between the two populations, a
weighting The process of weighting involves emphasizing the contribution of particular aspects of a phenomenon (or of a set of data) over others to an outcome or result; thereby highlighting those aspects in comparison to others in the analysis. That i ...
approach is used. Older groups in the Indigenous population are weighted more heavily (to match their prevalence in the "reference population", i.e. the overall Australian population) and younger groups less heavily. This gives an "age-adjusted"
morbidity A disease is a particular abnormal condition that negatively affects the structure or function of all or part of an organism, and that is not immediately due to any external injury. Diseases are often known to be medical conditions that a ...
rate approximately 30% higher than that for the general population, indicating that Indigenous Australians have a higher risk of circulatory disease. (Note that some residual distortion remains due to the wide age bands being used.) This is directly analogous to the standardized mortality ratio for mortality statistics. To adjust for age under this direct method of standardization, age-specific rates in each group must be calculated, as well as the age structure of the standard population.


Standard populations

In order to adjust for age, a standard population must be selected. Some agencies which produce health statistics also publish standard populations for age adjustment. Standard populations have been developed for specific countries and regions. World standard populations have also been developed to compare data from different countries, including the Segi World Standard and the
World Health Organization The World Health Organization (WHO) is a specialized agency of the United Nations responsible for international public health. The WHO Constitution states its main objective as "the attainment by all peoples of the highest possible level of h ...
(WHO) standard. These agencies must balance between setting weights which may be used over a long period of time, which maximizes comparability of published statistics, and revising weights to be close to the current age distribution. When comparing data from a specific country or region, using a standard population from that country or region means that the age-adjusted rates are similar to the true population rates. On the other hand, standardizing data using a widely used standard such as the WHO standard population allows for easier comparison with published statistics.


When is it used

Age adjustment is commonly used when comparing
prevalence In epidemiology, prevalence is the proportion of a particular population found to be affected by a medical condition (typically a disease or a risk factor such as smoking or seatbelt use) at a specific time. It is derived by comparing the number o ...
s in different populations. It is not used to derive life expectancy, which is calculated directly from the age-specific mortality rates, with no reference population required. Age adjustment is also ''not'' appropriate when attempting to compare population totals (for instance, if we wanted to know the total number of hospital beds required for patients with circulatory diseases).


See also

* *


References


Further reading

* {{DEFAULTSORT:Age Adjustment Epidemiology Design of experiments Demography