Internal and External Validity
General Issues

Developed by: W. Huitt, J. Hummel, D. Kaeck
Last revised: January, 1999

Internal Validity

One of the keys to understanding internal validity (IV) is the recognition that when it is associated with experimental research it refers both to how well the study was run (research design, operational definitions used, how variables were measured, what was/wasn't measured, etc.), and how confidently one can conclude that the change in the dependent variable was produced solely by the independent variable and not extraneous ones. In group experimental research, IV answers the question, "Was it really the treatment that caused the difference between the means/variances of the subjects in the control and experimental groups?" Similarily, in single-subject research (e.g., ABAB or multiple baseline), IV attempts to answer the question, "Do I really believe that it was my treatment that caused a change in the subject's behavior, or could it have been a result of some other factor?" In descriptive studies (correlational, etc.) internal validity refers only to the accuracy/quality of the study (e.g., how well the study was run-see beginning of this paragraph).

In their classic book on experimental research, Campbell and Stanley (1966) identify and discuss 8 types of extraneous variables that can, if not controlled, jeopardize an experiment's internal validity.

External Validity

The extent to which a study's results (regardless of whether the study is descriptive or experimental) can be generalized/applied to other people or settings reflects its external validity. Typically, group research employing randomization will initially possess higher external validity than will studies (e.g., case studies and single-subject experimental research) that do not use random selection/assignment. Campbell and Stanley (cited in Isaac & Michael, 1971) have identified 4 factors that adversely affect a study's external validity.

Increasing Internal and External Validity

In group research, the primary methods used to achieve internal and external validity are randomization, the use of a research design and statistical analysis that are appropriate to the types of data collected, and the question(s) the investigator(s) is trying to answer. Single-subject experimental studies almost always have high internal validity because subjects serve as their own controls but, as mentioned earlier, are extremely low with respect to external validity. Single-subject studies acquire external validity through the process of replication and extension (i.e., repeating the study in different settings, with a different subject, etc.). The results of group studies are also more acceptable by the scientific community when replicated.


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