Experimental Method - 5 Scientific Foundations of Psychology - STEP 4 Review the Knowledge You Need to Score High

5 Steps to a 5: AP Psychology - McGraw Hill 2021

Experimental Method
5 Scientific Foundations of Psychology
STEP 4 Review the Knowledge You Need to Score High

Some psychologists conduct experimental research in laboratories designed for carefully controlling conditions and measuring behavior.

“There’s a good probability that one of the essays on the AP exam will focus on research methods, especially experiments.”

—Elliott S., AP Teacher

The Controlled Experiment

The laboratory is one of the places where scientists test hypotheses, predictions of how two or more factors are likely to be related. Variables are factors that can have different values. In a scientific experiment, the researcher systematically manipulates a variable under controlled conditions and observes the response. The factor the researcher manipulates is called the independent variable (IV). The dependent variable (DV) is the behavior or mental process that is being measured, the factor that may change as a result of manipulation of the independent variable. If the dependent variable changes when only the independent variable is changed, the researcher can conclude that the change in the independent variable caused the change in the dependent variable. Thus, the independent variable is the cause, and the dependent variable is the effect. A controlled experiment is the only research method that can establish a cause and effect relationship.


An effective way to determine the independent and dependent variables is to word the hypothesis in the form of an “If . . . , then . . .” statement. What follows the “if ” is the independent variable (cause), and what follows the “then” is the dependent variable (effect). For example, “If students study for a quiz before going to sleep, rather than in the morning, then they will get higher scores on the quiz.” Studying for a quiz before going to sleep, rather than in the morning, is the independent variable and cause. Getting a higher score on the quiz is the dependent variable and effect.

For example, an experimenter hypothesizes that sleeping after studying for a biology quiz in the evening is more effective than studying for the same amount of time after waking in the morning. The population includes all the individuals in the group to which the study applies (all the students enrolled in introductory biology courses at the university for this example). To save time and money, most researchers use a subgroup of the population called a sample in their experimental research. The larger the sample size, the more likely it is to represent the population. The sample must fairly represent the whole group. This is achieved when every member of the population has an equal chance of being selected for the sample, when participants are selected randomly. Random selection can be achieved by putting all the names in a hat and picking out a specified number of names, by alphabetizing the roster of enrollees and choosing every fifth name, or by using a table of random numbers to choose participants. These are examples. To test the hypothesis, the scientist needs to randomly assign some subjects to an experimental group that receives the treatment and to randomly assign others to a control group that does not receive the treatment. The control group is a comparison group. This is called a between-subjects design because the participants in the experimental and control groups are different individuals. Everything is similar between the experimental group and the control group except for the independent variable. Random assignment of participants to the experimental and control groups minimizes the existence of preexisting differences between the two groups. Differences between the experimental group and the control group other than those resulting from the independent variable are called confounding variables. Confounding variables limit confidence in research conclusions. All participants, also called subjects, attend the same two sessions upon which the quiz is based. The experimental group is permitted to study for the quiz for 1 hour in the evening before going to sleep while the control group watches an unrelated comedy show. The control group studies for the quiz for 1 hour in the morning after awakening. The experimental group watches the comedy show in the morning at the same time. Everyone eats breakfast together, and then 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. Participants 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, and so forth.

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 whether 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 group 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.