Contextual Threats to Internal Validity
Reactivity of experimental arrangements
As a threat to external validity, reactivity of an experimental arrangement refers to the influence of the subjects' awareness that they are participating in an investigation. The results of an experiment may be influenced by the fact that subjects know they are being studies or that the purpose is to examine a particular outcome. The external validity question is whether the results would be obtained if subjects were not aware that they were being studied.
Multiple-treatment interference
In some experimental designs, subjects are exposed to more than one experimental condition. Subjects might receive two or more different interventions or alternate between intervention and no-intervention conditions. Multiple-treatment interference refers to drawing conclusions about a given treatment when it is evaluated in the context of other treatments. The conclusion drawn about one treatment or intervention might be restricted by the administration of prior treatments.
Novelty effects
As a threat to external validity, novelty effects refer to the possibility that the effects of an intervention may in part depend upon their innovativeness or novelty in the situation. It is possible that the effects of the intervention depend upon the fact that it is administered under conditions where it is particularly salient, infrequent, or otherwise novel in some way. The presence of novelty effects is difficult to evaluate. Thus, a new treatment when first proposed may seem to be very effective. Changes in the effects of an intervention over time might be due to the novelty of the early applications in the context of other available interventions.
