Discuss the various threats to internal and external validity.

that validity discussed here is in the context of experimental design, not in the context of measurement.

Internal validity refers specifically to whether an experimental treatment/condition makes a difference or not, and whether there is sufficient evidence to support the claim.

External validity refers to the generalizibility of the treatment/condition outcomes.

Factors which jeopardize internal validity

• History--the specific events which occur between the first and second measurement.
• Maturation--the processes within subjects which act as a function of the passage of time. i.e. if the project lasts a few years, most participants may improve their performance regardless of treatment.
• Testing--the effects of taking a test on the outcomes of taking a second test.
• Instrumentation--the changes in the instrument, observers, or scorers which may produce changes in outcomes.
• Statistical regression--It is also known as regression to the mean. This threat is caused by the selection of subjects on the basis of extreme scores or characteristics. Give me forty worst students and I guarantee that they will show immediate improvement right after my treatment.
• Selection of subjects--the biases which may result in selection of comparison groups. Randomization (Random _______ _______ ______ ________ ________ ____ ________ ____.
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