Definition
Effect modification occurs when an effect modifier is associated with both an apparent case and an apparent effect and modifies the association of interest (Katz 2001).
Consider the example of the association between vigorous exercise and risk for heart disease. This association is real, but its direction varies with level of fitness. An individual who is essentially fit will in all likelihood reduce his or her risk of heart disease by exercising vigorously. However, an individual who is unfit may acutely increase his or her risk by participating in such exercise, or by engaging in other vigorous physical activity. An unfortunately too frequent example of this occurs when unfit individuals attempt to shovel heavy snow, a very physically demanding task, and suffer a myocardial infarction. The association between vigorous exercise and heart disease, therefore, while real, is not unidirectional, but is modified by an individual’s level of physical fitness.
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References and Further Reading
Katz, D. L. (2001). Clinical epidemiology and evidence-based medicine: Fundamental principles of clinical reasoning and research. Thousand Oaks: Sage.
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Turner, J.R. (2020). Effect Modification. In: Gellman, M.D. (eds) Encyclopedia of Behavioral Medicine. Springer, Cham. https://doi.org/10.1007/978-3-030-39903-0_1018
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DOI: https://doi.org/10.1007/978-3-030-39903-0_1018
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