Free 5 Steps to a 5 AP Psychology, 2010-2011 Edition by Laura Lincoln Maitland Page A

they all take the same quiz. If the experimental group scores significantly higher than the control group, the experimenter can say that the results support the hypothesis. How does the experimenter measure effectiveness of studying? The experimenter uses the score on the quiz as the operational definition of effectiveness of studying. An operational definition describes the specific procedure used to determine the presence of a variable.
    In order to attribute a particular result to a specific factor, the controlled experiment must limit variables. Confounding variables that could contribute to the effect must be eliminated. Subjects in the biology quiz experiment need to share the same environmental factors; they need to eat the same foods, sleep in similar beds in the same rooms, sleep for the same amount of time, etc.
Eliminating Confounding Variables
    Experimenter bias (also called the experimenter expectancy effect ) is a phenomenon that occurs when a researcher’s expectations or preferences about the outcome of a study influence the results obtained. This is a special kind of confounding variable that can occur when a researcher is unaware that he or she is treating either the experimental group or control group differently from the other. A simple smile when addressing the experimental group that is not also shown to the control group qualifies as experimenter bias and as a confounding variable. The clues participants discover about the purpose of the study, including rumors they hear about the study suggesting how they should respond, are called demand characteristics . To eliminate the effects of demand characteristics, experimenters use the single-blind procedure , a research design in which the participants don’t know which treatment group—experimental or control—they are in. To eliminate the effects of both experimenter bias and demand characteristics, experimenters use the double-blind procedure , a research design in which neither the experimenter nor the participants know who is in the experimental group and who is in the control group. The double-blind is most easily accomplished when a second experimenter or assistant who doesn’t know the hypothesis or group assignments administers the experiment, keeping the principal investigator away from the subjects. When a number of factors might be responsible for an observed effect, to determine which deserves the credit, an experimenter needs to systematically manipulate or vary one or more factors while holding constant all the others that might be important. The effects of these manipulated events on some behavioral reaction are then assessed. It is then possible to demonstrate whether one factor is responsible for the result or if an interacting package of factors is involved.
    In experiments involving drugs, participants in the experimental group usually receive the drug with the active ingredient, while subjects in the control condition receive a drug that seems identical, but lacks the active ingredient. The imitation pill, injection, patch, or other treatment is called a placebo . Subjects sometimes believe that the treatment will be effective, and they think they experience an improvement in health or well-being. This is the placebo effect . The placebo effect is now used to describe any cases when experimental participants change their behavior in the absence of any kind of experimental manipulation. The experiments need not involve drugs at all.
    A research design that uses each participant as his or her own control is called a within-subjects design . For example, the behavior of an experimental participant before receiving treatment might be compared to his or her behavior after receiving treatment. Two treatmentsmight be tried. If two treatments are used, the order of the treatments could cause an effect. To eliminate the possibility, psychologists use counterbalancing , a procedure that assigns half the subjects to one of the treatments first