Internal validity occurs when a researcher controls all extraneous variables and the only variable influencing the results of a study is the one being manipulated by the researcher. This means that the variable the researcher intended to study is indeed the one affecting the results and not some other, unwanted variables. (Take a look at the definition for “confound”) There are several “threats to internal validity” including: history, maturation of participants, testing, instrument decay, and statistical regression.
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