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| EPI2-9: Crude and Specific Measures |
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| EPI2-9: Crude and Specific Measures |
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OBJECTIVES |
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At the end of this session you should understand the differences between crude measures of occurrence and stratum-specific measures of occurrence. |
The measures of disease occurrence discussed above may be calculated for a whole population or calculated separately for subgroups of the population.
In the first case the measures are called crude measures and in the latter case specific measures.
For example, if incidence rates are calculated for different age groups within a population, they are referred to as age-specific incidence rates.
Subgroups or strata are usually different age groups or, in workplaces, different exposure groups.
A population is divided into subgroups, called strata, when there is reason to believe that the occurrence of the disease may vary from one group to another.
In occupational health we are very interested in seeing if exposure causes disease. The occurrence of disease will be greater in higher exposure strata if exposure causes disease.
The stratum specific measures may be of interest in themselves, but what they tell us may remain hidden if only crude measures are assessed. The magnitude of a crude measure for a population depends not only on the magnitude of the stratum specific measures that apply to subgroups of the population, but also on the way the population is distributed over the different subpopulations in the strata.
One year the crude mortality rate (number of deaths divided by the mean population size during the year) in a developed country was 0.010 per year while in a less developed country it was only 0.008 per year.
The explanation for the difference is not that it was riskier to live in the more developed country.
On further examination, all age-specific mortality rates, except those for the very oldest age category, were higher in the less developed country compared with the developed country.
The explanation is that a greater proportion of the developed country population was in the older age categories where the age-specific mortality rates are higher.
So the crude or overall mortality rate was paradoxically higher in the developed country - because the developed country population is older than the one in the developing country.
This will be covered in greater detail under STANDARDISATION.
