In formulating hypotheses, a common error is the uncritical acceptance of apparent commonsense but untested assumptions. Thus in the field of psychology, it was for many years automatically assumed that the main cause of forgetfulness is the interval of time elapsing between successive exposures to a learning stimulus. Experimentation, however, was subsequently undertaken, and several other factors, such as motivation and the strength or effectiveness of the stimulus, turned out to have an ever more important bearing on the problem. A somewhat similar error arises from neglect of multiple causes. Thus two events may be found to be associated, e.g. when the incidence of a disease in a smoky industrial sector of a city is significantly higher than that in the smoke-free zones, a research worker might infer that the existence of the disease is due to the smokiness of the area when in fact it might equally well be found in other reasons, such as the under-nourishment of the inhabitants or over-crowding.
