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Unlike correlational research methods or psychological tests, experiments can provide information about cause-and-effect relationships between variables. In an experiment, a researcher manipulates or changes a particular variable under controlled conditions while observing resulting changes in another variable or variables. The researcher manipulates the independent variable and observes the dependent variable. The dependent variable may be affected by changes in the independent variable. In other words, the dependent variable depends (or is thought to depend) on the independent variable.
Experimental and Control Groups
Typically, a researcher conducting an experiment divides participants into an experimental group and a control group. The participants in both groups receive the same treatment, with one important difference: the researcher manipulates one part of the treatment in the experimental group but does not manipulate it in the control group. The variable that is manipulated is the independent variable. The researcher can then compare the experimental group to the control group to find out whether the manipulation of the independent variable affected the dependent variable.
Often, participants in the control group receive a placebo drug or treatment, while participants in the experimental group receive the real drug or treatment. This helps researchers to figure out what causes the observed effect: the real drug or treatment, or the participants’ expectation that they will be affected.
Example: Suppose a researcher wants to study the effect of drug A on participants’ alertness. He divides 100 participants into two groups of 50, an experimental group and a control group. He dissolves drug A in saline solution and injects it into all the participants in the experimental group. He then gives all the control group participants an injection of only saline solution. The independent variable in this case is drug A, which he administers only to the experimental group. The control group receives a placebo: the injection of saline solution. The dependent variable is alertness, as measured by performance on a timed test. Any effect on alertness that appears only in the experimental group is caused by the drug. Any effect on alertness that appears in both the experimental and control groups could be due to the participants’ expectations or to extraneous variables, such as pain from the injection.
Extraneous Variables
Ideally, participants in the experimental and control groups would be identical in every way except for the variables being studied. In practice, however, this would be possible only if researchers could clone people. So researchers try to make groups with participants that are similar in all respects that could potentially influence the dependent variable. Variables other than the independent variable that could affect the dependent variable are called extraneous variables.
One way to control extraneous variables is to use random assignment. When researchers use random assignment, they create experimental and control groups in a way that gives participants an equal chance of being placed in either group. This guarantees the two groups’ similarity.
Disadvantages of Experiments
The main disadvantage of experiments is that they usually don’t fully reflect the real world. In an experiment, researchers try to control variables in order to show clear causal links. However, to exert control in this way, researchers must simplify an event or a situation, which often makes the situation artificial.
Another disadvantage of experiments is that they can’t be used to study everything. Sometimes researchers can’t control variables enough to use an experiment, or they find that doing an experiment would be unethical—that is, it would be painful or harmful in some way to the participants being studied.
