As diseases are usually multifactorial in causation, several factors (confounders) will participate in a web fashion (indirectly, cumulatively, combinedly, complementing etc.) and they will be operating both in cases and controls.

If they are not neutralized both in cases and controls and the confounding bias is allowed to continue, the meaning of comparison between cases and controls looses its significance, thereby the validity of the study

HENCE, IT IS IMPERATIVE TO FIND OUT ALL THE CONFOUNDING VARIABLES PLAYING A ROLE IN CAUSAL MECHANISMS AND ELIMINATE OR NEUTRALIZE THEM BEFORE ACCEPTING CAUSAL ASSOCIATION.