Once equivalent groups have been formed, the experimenter tries to treat them identically except for the one "variable" that he or she wishes to isolate. Human experimentation requires special safeguards against outside variables such as the "placebo effect". Such experiments are generally "double blind", meaning that neither the volunteer nor the researcher knows which individuals are in the control group or the experimental group until after all of the data have been collected. This ensures that any effects on the volunteer are due to the treatment itself and are not a response to the knowledge that he is being treated.