| Causal relationship | A relationship in which changes to the value of one variable cause changes in the value of another.
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| Correlational relationship | A relationship in which the value of one variable changes systematically with the value of a second variable.
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| Correlational research | Research in which no independent variables are manipulated. Instead, two or more dependent variables are measured to identify possible correlational relationships.
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| Third-variable problem | A problem that interferes with drawing causal inferences from correlational results. A third, unmeasured variable affects both measured variables, causing the latter to appear correlated even though neither variable influences the other.
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| Directionality problem | A problem that interferes with drawing causal inferences from correlational results that involves not being able to clearly specify the direction of causality between variables.
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| Experimental research | Research in which independent variables are manipulated and behavior is measured while extraneous variables are controlled.
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| Independent variable | The variable that is manipulated in an experiment. Its value is determined by the experimenter, not the subject.
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| Treatment | A level of an independent variable applied during an experiment. In multifactor designs, a specific combination of the levels of each factor.
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| Dependent variable | The variable measured in a study. Its value is determined by the behavior of the subject and may depend on the value of the independent variable.
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| Experimental group | A group of subjects in an experiment that receives the nonzero level of the independent variable.
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| Control group | A group of subjects in an experiment that does not receive the experimental treatment. The data from the control group are used as a baseline against which data from the experimental group are compared.
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| Extraneous variable | Any variable that is not systematically manipulated in an experiment but that still may affect the behavior being observed.
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| Demonstration | A nonexperimental technique in which some phenomenon is demonstrated. No control group is used.
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| Internal validity | The extent to which a study evaluates the intended hypotheses.
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| Confounding | Two variables that vary together in such a way that the effects of one cannot be separated from the effects of the other.
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| External validity | The extent to which the results of a study extend beyond the limited sample used in the study.
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| Simulation | A laboratory research technique in which you attempt to re-create as closely as possible a real-world phenomenon.
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