The second criterion requires that the cause and effect covary. For example, when no force is applied to a tendon, it remains in a relaxed state; in the presence of the cause (a force), the tendon responds.
The third criterion involves the absence of other plausible explanations for the observed effect. To the extent that confounding factors have been controlled by the design of the experiment or observation, other explanations for the observed effect are less likely. In some studies, it is possible to use random assignment of participants to conditions to control the influence of other factors, but this is not the only means for achieving control. In making a determination of whether factors other than the experimentally manipulated factor (e.g., ergonomic redesign) offer plausible explanations for the observed effect (e.g., a lower average level of sick days in the post-redesign period of observation), it is necessary to identify and test whether other plausible factors might have been operative, mimicking the effect of the target cause (e.g., change in sick leave policy, turnover in personnel). If no other such causes can be shown empirically to be responsible for the effect, it is reasonable to attribute the effect to the cause under investigation. Note that this criterion stipulates that other causal factors need to be plausible, not merely logically possible. Claims of plausibility have merit to the extent that they can be empirically supported.
The fourth criterion, temporal contiguity, amplifies the first (temporal ordering). To the extent that the effect follows the cause closely in time, the plausibility that other factors are operative is reduced. For example, if a tendon reacts immediately to the presence of force, it is unlikely that other factors (e.g., gravitational pull) are responsible for the sudden elongation. On the other hand, if there is a delay between the application of force and the response that cannot be explained by the biomechanical mechanisms associated with tendon structures, other factors may be operative, weakening the strength of the causal claim.
The fifth criterion is that the size of the cause is related to the size or magnitude of the effect: that is, there is a congruity between the cause and effect. More specifically, a small force or small change in the workplace ought to correspond to a small effect and a large force or major change (e.g., multiple components of the workplace are altered) ought to be accompanied by a large effect (e.g., failure of a tissue or a substantial reduction in work-related injuries). When results from experiments or quasi-experiments violate these expectations, it is necessary to examine how the effect was modified (either enhanced, in the case of a small cause that leads to a large effect, or dampened, in the case of a large cause that produces a small effect). To the extent that a compelling explanation for these anomalies cannot be provided (e.g., delays in the implementation of a workplace redesign), it is plausible to assume that other processes (not related to the suggested cause) are responsible for some or all of the effect.
By applying these criteria to evaluate the credibility of scientific evidence, one need not place heavy emphasis on the types of research design that have been used in a given study. It is a disciplined way to take advantage of the research provided by a wide variety of methods and, thus, it has substantial implications for the manner in which a science base is considered. Rather than focusing on a study's design features, one considers the pattern of data for each study and its associated design on a case-by-case basis. For some studies, it is readily apparent that even the minimal causal criteria cannot be substantiated (e.g., it is impossible to establish that the cause came before the effect). For others, even when there is no conventional experimental control group (created through random assignment), observations before and after the introduction of an