• Understanding Research: Confounding

    by Cian O'Brien. Last modified: 20/04/14



    • Confounding involves the possibility that an observed association is due totally/in part to the effects of differences between the study groups that could affect their risk of developing the outcome being studies
    • Confounding occurs when the effects of 2 associated exposures have not been separated, resulting in the interpretation that the effect is due to one variable rather than the other
    • The consequence of confounding is that the estimated association is not the same as the true effect

    In order for a variable to be considered as a confounder:

    1. The variable must be independently associated with the outcome (be a risk factor)
    2. The variable must be associated with the exposure under study in the source population
    3. It should not lie on the causal pathway between exposure and disease

    Effects of confounding

    • Confounding factors, if not controlled for, cause bias in the estimate of the impact of the exposure being studies

    The effects of confounding can result in:

    • An observed difference between study populations when no real difference exists
    • No observed difference between study population when a true association does exist
    • An underestimate of an effect
    • An overestimate of an effect

    Residual Confounding

    • Residual confounding occurs when a confounder has not been adequately adjusted for in the analysis ( by using too large age groups)


    • A study finds alcohol consumption to be associated with the risk of Coronary Heart Disease. However, smoking may have confounded the association between alcohol and CHD. For example smoking is independently associated with CHD (is a risk factor) and is also associated with alcohol consumption (smokers tend to drink more than non-smokers).
    • Controlling for the potential confounding effect of smoking may in fact show no association between alcohol consumption and CHD.


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    Cian O'Brien

    Cian O'Brien

    Cian is an Irish trained Emergency Medical Technician, Registered General Nurse and holds a Masters degree in Public Health from University College Cork, Ireland. His research interests include prehospital care and marathon medicine.

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