In experimental research, the investigator manipulates conditions for the purpose of determining their effect on behavior. Subjects should be unaware of their membership in an experimental group so that they don’t act differently (Hawthorne Effect). In the simplest experimental design, investigators administer a placebo to the control group and a treatment to the experimental group. Experimental designs vary in terms of subjects’ assignments to different groups, whether subjects were pre-tested, whether different treatments were administered to different groups, and the number of variables being investigated.