Internal validity is a form of experimental validity (1). An experiment is said to possess internal validity if it properly demonstrates a causal relation between two variables (2,3). An experiment can demonstrate a causal relation by satisfying three criteria:

1. That the "cause" precedes the "effect" in time (temporal precedence),
2. that the "cause" and the "effect" are related (covariation), and
3. that there are no plausible alternative explanations for the observed covariation (nonspuriousness) (4).

In scientific experiments, researchers often manipulate a variable (the independent variable) to see what effect it has on a second variable (the dependent variable) (5). For example, a researcher might manipulate the dosage of a particular drug to see what effect it has on a person's health. If the experiment allows the researcher to conclude that different doses of the drug caused a change in peoples' health (by satisfying the above criteria), then the study possesses internal validity.

In other words, an experiment possesses internal validity if the observed changes in the dependent variable were caused by the manipulation of the independent variable.

Threats to Internal Validity

• Ambiguous Temporal Precedence
• Selection Bias
• Historical Change
• Participant Maturation
• Regression to the Mean
• Participant Mortality
• Testing Effects
• Instrumentation Change

See also

• External Validity
• Construct validity
• Content validity
• Statistical conclusion validity
• Ecological validity

References

1. Mitchell, M. and Jolley, J. (2001). Research Design Explained (4th Ed) New York:Harcourt.
2. Brewer, M. (2000). Research Design and Issues of Validity. In Reis, H. and Judd, C. (eds.) Handbook of Research Methods in Social and Personality Psychology. Cambridge:Cambridge University Press.
3. Shadish, W., Cook, T., and Campbell, D. (2002). Experimental and Quasi-Experimental Designs for Generilized Causal Inference Boston:Houghton Mifflin.
4. ibid.
5. Levine, G. and Parkinson, S. (1994). Experimental Methods in Psychology. Hillsdale, NJ:Lawrence Erlbaum.

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