EXPERIMENTAL RESEARCH METHODS

EXPERIMENTAL RESEARCH METHODSJohn Davis, Ph.D.

Part Two: Types of Variables and Validity

Contents
TYPES OF VARIABLES
RELIABILITY AND VALIDITY

Types of Variables 1. Independent Variable (IV): IV has levels, conditions, or treatments. Experimenter may manipulate conditions or measure and assign subjects to conditions; supposed to be the Cause In the example, it is the psychotherapy. 2. Dependent Variable (DV): measured by the experimenter; the Effect or result. In the example, it is the mental health of the participants. 3. Control Variables: held constant by the experimenter to eliminate them as potential causes. For instance, if I use only research participants who have been problems with anxiety or depression, this diagnosis would be a control variable. 4. Random Variables: allowed to vary freely to eliminate them as potential causes. Many other characteristics of the research participants, as long as they really do vary freely. Examples might include age, personality type, or career goals. 5. Confounding Variables: vary systematically with the independent variable; may also be a cause. Good experimental designs eliminate them. Say I divide the research participants into two groups, one of which gets the new psychotherapy (the experimental group) and one of which does not (the control group). If there is some systematic difference between these two groups, it will not be a fair test. If those in the psychotherapy group know they are getting a new treatment and therefore expect to get better while those in the control group know they are not getting any treatment and expect to get worse, the expectations will be a confounding variable. If the experimental group does improve, we will not know whether it was because of the psychotherapy itself (the Independent Variable) or because of the participants' expectations (a Confounding Variable). Go to TOP of this page RELIABILITY AND VALIDITY 1. RELIABILITY Are the results of the experiment repeatable? If the experiment were done the same way again, would it produce the same results? Reliability is a requirement before the validity of the experiment can be established. 2. INTERNAL VALIDITY Accuracy or truth-value Does the research design lead to true statements? Did the independent variable cause the effects in the dependent variable? In experimental research, this usually means eliminating alternative hypotheses. In the example evaluating a new psychotherapy, the issue of internal validity is whether the psychotherapy really was the causal factor in improving participants' mental health. 3. EXTERNAL VALIDITY Generalizability Can the results can be applied in another setting or to another population of research participants? Go to TOP of this page Go to PREVIOUS page, the first on Experimental Research Methods Go to NEXT page

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