association is present or to a finding of no association when in fact one exists. An apparent effect of deployment on a health outcome might be the result of random variation due to sampling of the study population rather than the result of exposure to the agent. Standard methods that use confidence intervals, for example, allow one to assess the role of chance variation due to sampling.

Multiple Comparisons

When an investigator initiates a large number of analyses simultaneously on the same dataset, multiple comparisons pose a problem. When looking at so many different comparisons, the investigator is bound to find something of note by chance alone. For example, in many Gulf War veteran studies, the investigators are comparing multiple outcomes and multiple exposures. There are, however, ways to correct for multiple comparisons in studies. One way is to use a Bonferroni correction, a statistical adjustment for multiple comparisons. It effectively raises the standard of proof needed when an investigator looks at a wide array of hypotheses simultaneously.

LIMITATIONS OF GULF WAR VETERAN STUDIES

The epidemiologic and clinical studies to date have provided valuable information regarding the health of Gulf War veterans; however, many of the studies have limitations that hinder accurate assessment of the veterans’ health status. The limitations include the possibility that study samples do not represent the entire Gulf War population, the relatively young age of the exposed population, low rates of participation in studies, reinforcement of self-reporting of symptoms and exposures, insensitivity of instruments for detecting abnormalities in deployed veterans, and a period of investigation that is too brief to detect health outcomes that have long latency, such as cancer. In addition, many of the US studies are cross-sectional and this limits the opportunity to learn about symptom duration and chronicity, latency of onset, and prognosis. Finally, the problem of multiple comparisons that is common in many of the Gulf War studies results in confusion over whether the effect is real or occurring by chance. Those limitations make it difficult to interpret the results of the findings, particularly when several well-conducted studies produce inconsistent results.

REFERENCES

Evans, A. S. 1976. Causation and disease: The Henle-Koch postulates revisited. Yale Journal of Biology and Medicine 49(2):175-195.

Hill, A. B. 1965. The environment and disease: Association or causation? Proceedings of the Royal Society of Medicine 58:295-300.

IOM (Institute of Medicine). 2000. Gulf War and Health, Volume 1: Depleted Uranium, Sarin, Pyridostigmine Bromide, Vaccines. Washington, DC: National Academy Press.

IOM. 2003. Gulf War and Health, Volume 2: Insecticides and Solvents. Washington, DC: The National Academies Press.

IOM. 2005. Gulf War and Health, Volume 3: Fuels, Combustion Products, and Propellants. Washington, DC: The National Academies Press.



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