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The results of epidemiological studies obviously depend on the comparisons made between the cases and control groups used. If the groups are not well matched, the results will not be meaningful. For this reason cases and control groups may be matched for age, sex and race, as well as lifestyle factors like smoking and alcohol consumption. Occupational groups very often have lower total mortality than the general population as the latter includes people unable to work due to illness or disability. In other words, any group of workers is likely to be more healthy than the population as a whole, a phenomenon known as the "healthy worker effect" (Encyclopedia of occupational health and safety, ILO, 1998).

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