Confounding bias is caused by the presence of an extraneous factor associated with both an exposure under study and a disease outcome. A commonly used method to adjust for a potential confounder is stratification in which the comparison between exposure and disease is done at specific levels of the potential confounder. When a study mentions that they controlled for a factor, they have tried to remove the effect of that variable. While confounding is a bias which an investigator wishes to eliminate, effect modification refers to a difference in the magnitude of an effect measure across levels of another variable. An example is throat cancer (disease) and high alcohol use (risk factor). An effect modifier is smoking, since the relative risk for alcohol has been reported to be greater at higher levels of smoking. More accurately this should be called risk-ratio modification since effect modification also depends on the risk scale for the outcome, for example risk ratio or risk difference.
If you don't know whether an effect is caused by the variable you are interested in (e.g. a drug or smoking) or by another variable (e.g. age or sex) then the other variable is called a confounder and it is said to cause confounding. A confounder is a variable other than the one being investigated which is asociated with both the exposure and the outcome. A confouder can cause bias in a study.
A condition or variable that may be a factor in producing the same response as the substance under study. The effects of such factors may be discerned through careful design and analysis.
A variable that is associated with an exposure or treatment of interest and, as a result, influences the relationship between the exposure or treatment and an outcome. The ability to adjust or analytically control for the presence of a confounder depends on how well this variable is measured.
Factors that distort or mask the true effect of exposure in an epidemiologic study.
a characteristic of the user, which distorts the risk associated with exposure to a particular therapy because in itself it could increase the risk of the condition under scrutiny
Variable that influences a health effect apart from air pollution. In particular, a confounder is associated with the exposure and the outcome and effect estimates would be biased if the variable would be neglected in the analyses.
a factor that can influence a trial or experiment, leading to unexpected outcomes that improperly skew the results.
a variable that affects the observed relation between 2 other variables (eg, alcohol is related to lung cancer, but does not cause the disease; instead, both alcohol and lung cancer are related to smoking, and it is the smoking that causes lung cancer).
A condition or variable that is both a risk factor for disease and associated with an exposure of interest. This association between the exposure of interest and the confounder (a true risk factor for disease) may make it falsely appear that the exposure of interest is associated with disease.