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When measuring signals at different points on the skin of the body, you don't usually want to connect the <s>victim</s> patient to ground.  Usually you use an extra electrode connected some distance away that is driven by the circuitry to the average of the other signals.  That reduces the common mode range of the other signals, and cancels out some of the common mode signal.  This is often called the "right leg driver" for EKGs.  EEGs don't do it exactly like that, but the general concept is still valid.