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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 treatments might 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 and the other half of the subjects to the other treatment first.

Quasi-Experimental Research

Quasi-experimental research designs are similar to controlled experiments, but participants are not randomly assigned. Experimental research designs to study differences in behavior between men and women, boys and girls, young and old, or students in one class and students in another class are “sort of” experiments or quasi-experiments. Because of confounding variables—preexisting differences between the experimental group and comparison groups—quasi-experiments do not establish cause and effect relationships, although they can point in the direction of them.

Correlational Research

Although experiments conducted under carefully controlled conditions help establish cause and effect relationships, the time, expense, and artificiality of the environment limits that type of research. Psychologists more often use descriptive and correlational research methods such as survey methods that involve interviews or questionnaires, tests, and naturalistic observation. Correlational methods look at the relationship between two variables without establishing cause and effect relationships. The goal is to determine to what extent one variable predicts the other. Many factors that seem to be causally related are not. Often it’s a third factor that causes the other two.

Naturalistic Observation