Reflecting the fact that two or more variables that might have caused an effect were simultaneously present, so that we do not know to which to attribute the effect, confounding variable.
The combined effect of the factor under investigation and other factors. A factor can only be a confounder if the occurrence of the disease as well as the exposure under investigation is associated with it. There is an essential difference between confounding and information or selection bias. If information on the confounder is collected during the study, it can be adjusted for in the statistical analyses. Sometimes, however, an unknown or not measured confounder is present. Such a confounder cannot be adjusted for in the statistical analyses and gives rise to bias in the study results.