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"A threat to external validity is an explanation of how you might be wrong in making a generalization from the findings of a particular study." [5] In most cases, generalizability is limited when the effect of one factor (i.e. the independent variable) depends on other factors.
A major factor in this is whether the study sample (e.g. the research participants) are representative of the general population along relevant dimensions. Other factors jeopardizing external validity are: Reactive or interaction effect of testing, a pretest might increase the scores on a posttest
The experimental design may also identify control variables that must be held constant to prevent external factors from affecting the results. Experimental design involves not only the selection of suitable independent, dependent, and control variables, but planning the delivery of the experiment under statistically optimal conditions given the ...
They are also applicable to a system of multiple equations, in which X (and other factors) affect Y through several intermediate variables. An instrumental variable need not be a cause of X ; a proxy of such cause may also be used, if it satisfies conditions 1–5. [ 10 ]
A distinction of sampling bias (albeit not a universally accepted one) is that it undermines the external validity of a test (the ability of its results to be generalized to the rest of the population), while selection bias mainly addresses internal validity for differences or similarities found in the sample at hand. In this sense, errors ...
There are five key principles relating to internal validity (study design) and external validity (generalizability) which rigorous impact evaluations should address: confounding factors, selection bias, spillover effects, contamination, and impact heterogeneity. [5]
External validity pertains to the process of generalizing the findings of the study to the population from which the sample was drawn (or even beyond that population to a more universal statement). This requires an understanding of which conditions are relevant (or irrelevant) to the generalization.
Over time, as the novelty wears off, the stress response decreases. This is a threat to external validity when individuals participating in a research study (a novel situation) perceive and respond differently than they would in the normal real world. [2]