The committee’s process for reaching conclusions about TBI and its potential for adverse health outcomes was collective and interactive. Once a study was included in the review because it met the committee’s criteria, there were several considerations in assessing causality, including strength of the association, presence of a dose–response relationship, presence of a temporal relationship, consistency of the association, and biologic plausibility.

The committee attempted to express its judgment of the available data clearly and precisely. It agreed to use the categories of association that have been established and used by previous Committees on Gulf War and Health and other Institute of Medicine committees that have evaluated vaccine safety, effects of herbicides used in Vietnam, and indoor pollutants related to asthma (IOM, 2000, 2003, 2005, 2006, 2007). Those categories of association have gained wide acceptance over more than a decade by Congress, government agencies (particularly the Department of Veterans Affairs), researchers, and veterans groups.

The five categories below describe different levels of association and sound a recurring theme: the validity of an association is likely to vary to the extent to which common sources of spurious associations could be ruled out as the reason for the observed association. Accordingly, the criteria for each category express a degree of confidence based on the extent to which sources of error were reduced. The committee discussed the evidence and reached consensus on the categorization of the evidence for each health outcome in the various outcome chapters (Chapters 6–10).

Evidence is sufficient to conclude that there is a causal relationship between sustaining a TBI and a specific health outcome in humans. The evidence fulfills the criteria of sufficient evidence of an association (below) and satisfies several of the criteria used to assess causality: strength of association, dose–response relationship, consistency of association, temporal relationship, specificity of association, and biologic plausibility.

Evidence is sufficient to conclude that there is a positive association; that is, a consistent association has been observed between sustaining a TBI and a specific health outcome in human studies in which chance and bias, including confounding, could be ruled out with reasonable confidence as an explanation for the observed association.

Evidence is suggestive of an association between sustaining a TBI and a specific health outcome in human studies but is limited because chance, bias, and confounding could not be ruled out with reasonable confidence.