After the intervention, we take a posttest measurement (O 2). In many experimental designs, we take a pretestmeasurement (O 1) of the dependent variable before introducing the intervention (X). Historyĭuring the time that an experiment is taking place, some unanticipated and unplanned events may occur that confuse the relationship being studied. Good experimental techniques, in which the effect of an independent variable on a dependent variable is studied under highly controlled conditions, usually allow for higher degrees of internal validity than, for example, single-case designs.Ĭambell and Stanley (1963) and later Cook and Cambell (1979) listed eight kinds of confounding variables that can interfere with internal validity (i.e., with the attempt to isolate causal relationships). In other words, strong internal validity refers to the unambiguous assignment of causes to effects. Strong internal validity means that we have not only reliable measures of our independent and dependent variables but also a strong justification that causally links our independent variables to our dependent variables. In other words, internal validity addresses the ‘true’ causes of outcomes that we observe in our study. Internal validity refers to whether the experimental treatment was the sole cause of observed changes in the dependent variable.
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