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Epidemiological study in situations where nature is allowed to take its course. Changes or differences in one characteristic are studied in relation to changes or differences in others, without the intervention of the investigator.
A measure of association which quantifies the relationship between an exposure and health outcome from a comparative study; also known as the cross-product ratio.
Classification into ordered qualitative categories; e.g.
Synonymous with epidemic. Sometimes the preferred word, as it may escape sensationalism associated with the word epidemic.