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4 CONSIDERATIONS IN IDENTIFYING AND EVALUATING THE LITERATURE
Pages 103-116

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From page 103...
... The committee focused its attention on clinical and epidemiologic studies of adults with long-term health effects that resulted from a TBI by any mechanism, such as occupational injury, motor-vehicle collision, sports injury, gunshot wound, or other act of violence, including military combat injury. Studies of patients with a TBI due to malignancy, stroke, infection, ischemia, other diseases or disorders of the brain, intoxication, or oxygen deprivation were not considered.
From page 104...
... Animal Studies Studies of laboratory animals are essential for understanding mechanisms of action and biologic plausibility and for providing information about possible health effects when experimental research in humans is not ethically or practically possible (NRC, 1991)
From page 105...
... Spurious associations, that is, the finding of an association that does not truly exist, can be due to random error or chance, systematic error or bias, or a combination of them. Random error or chance is a statistical variation in a measurement taken from a sample of a population that can lead to the appearance of an association when none is present or the failure to find an association when one is present.
From page 106...
... ; that is, causality is inferred, rather than measured directly, in observational studies. In 1965, Austin Bradford Hill, a British statistician, suggested nine criteria that could be used to assess whether an association observed in an observational study might be causal (Hill, 1965)
From page 107...
... . It interpreted the lack of an appropriate sequence as evidence against causality but recognized that insufficient knowledge about the natural history and pathogenesis of many of the health effects under review limited the utility of this consideration.
From page 108...
... Those types of studies are valuable for determining risk factors other than TBI for specific outcomes, but they do not provide information on whether a particular outcome, such as Parkinson disease, is associated with TBI. Cohort studies can be used to estimate risk difference, RR, and hazards, all of which measure the strength of an association.
From page 109...
... Since military personnel are at overall lower risk of adverse health outcomes compared to the general population, any excess risk associated with an exposure they experience must be large enough to overcome their inherent advantage in order to be detectable by such methods as SMR. Case–Control Studies In a case–control study, subjects (cases)
From page 110...
... However, case–control studies are vulnerable to several types of bias, such as recall bias, in which cases are more likely to report exposures than controls, which can dilute or enhance an association between a health effect and an exposure. Other problems include identifying representative groups of cases, choosing suitable controls, and collecting comparable information on exposures in both cases and controls.
From page 111...
... Methodologic Rigor A study had to be a published in a peer-reviewed journal or other rigorously peerreviewed publication, such as a government report, dissertation, or monograph; include sufficient methodologic details to allow the committee to judge whether it met inclusion criteria; include an unexposed control or reference group; have sufficient statistical power to detect effects; and use reasonable methods to control for confounders. Exposure Assessment For a study to be considered primary, the committee preferred studies that had an independent assessment of a TBI rather than self-reports of a TBI or reports by family members.
From page 112...
... Sufficient Evidence of an Association 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. Limited/Suggestive Evidence of an 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.
From page 113...
... The possibility of a very small increase in risk of the health outcome after sustaining a TBI cannot be excluded. LIMITATIONS OF STUDIES Many of the studies reviewed by the committee presented substantial obstacles to determining associations between TBI and long-term health outcomes because they were beset by limitations that are commonly encountered in epidemiologic studies, including lack of representative sample, selection bias, lack of control for potential confounding factors, selfreports of exposure and health outcomes, and outcome misclassification.
From page 114...
... In such studies, attempts to examine even smaller subpopulations magnify the difficulties and reduce the likelihood of detecting meaningful differences. Of the studies examined by the committee, those with small samples were also sometimes hampered by other problems discussed above, including low participation rates, loss to followup, inadequate duration of followup, and self-reporting of symptoms.
From page 115...
... 2006. Gulf War and Health, Volume 4: Health Effects of Serving in the Gulf War.


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