There are two classes of factors that jeopardize the validity of research results: internal and external. Internal validity is the extent to which a test measures what it claims to measure. External validity, on the other hand, is the extent to which the findings of a research study can be generalized to other groups, times, and contexts (Trochim and Donnelly, 2008). Internal validity is threatened whenever the possibility exists that alternative causes, other than the independent variable, are responsible for the effect. There are numerous possible threats to internal validity; the seven most commonly cited threats include history, maturation, testing, instrumentation, mortality, regression, and selection. History refers to specific events, in addition to treatment, that occur between the first and second measurements. The longer the interval between the pre-test and post-test, the more viable this threat becomes. Maturation concerns changes in physical, intellectual, or emotional characteristics, which naturally occur over time, that influence the results of a research study. For example, in longitudinal studies, individuals age and become more sophisticated. Testing: Refers to the effects of taking one test on performing a second test. Exposure to the pretest can influence performance on a posttest. The shorter the interval between the pre-test and the post-test, the more viable this threat becomes. Instrumentation concerns changes in how a test or other measurement tool is calibrated that might take into account the results of a research study. This threat is more likely to occur due to an unreliable measurement instrument (Creswell, 2009). Mortality refers to the differential loss of an individual... middle of paper...4. Choosing an appropriate research design can help control most other threats to internal validity. In general, threats to a study's external validity can be minimized if the researcher has taken steps to ensure that the sample, setting, and context are representative of reality. the population, environment and context to which the findings are intended to be generalized (Trochim and Donnelly, 2008). Works Cited Creswell, J. W. (2009). Research design: Qualitative, quantitative, and mixed methods approaches (3rd ed.). Thousand Oaks, California: SAGE Publications, Inc. Fraenkel, J. R., & Wallen, N. E. (2006). How to design and evaluate education research (6th ed.). New York, New York: McGraw-Hill Humanities/Social Sciences/Languages.Trochim, W.M.K., & Donnelly, J.P. (2008). The knowledge base of research methods (3rd ed.). Mason, Ohio: Cengage Learning.