Validity of experiments

Experiments should possess both internal and external validity. Internal validity refers to whether the experiment treatment is solely accountable for the changes occurring or whether some extraneous factors were also at work. If there were outside factors as well, then internal validity is affected by the ability to measure their impact. External validity is to do with whether the findings can be applied to the real-world situation. Experiments can be conducted either in the field or under controlled laboratory conditions. In the former case the establishment of internal validity can be quite difficult. Laboratory experiments, by contrast, may lack external validity because of their controlled environment.


It is customary for tests or experiments involving new products to be carried out under laboratory conditions. Unfortunately, because in actual usage the operating conditions for the products may be very different to those in the laboratory, the external validity of such tests and experiments is difficult to assess.

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