. "2 Methodological Considerations in Evaluating the Epidemiologic Literature on Cancer and Exposure to Trichloroethylene." Assessing the Human Health Risks of Trichloroethylene: Key Scientific Issues . Washington, DC: The National Academies Press, 2006.
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Assessing the Human Health Risks of Trichloroethylene: Key Scientific Issues
A thorough search of the literature must be conducted to make sure that no relevant studies are overlooked. An important effort is to identify studies that could have investigated the risks associated with trichloroethylene.
An analysis of publication bias should be conducted.
As much as possible, all relevant studies should be included in the analysis. Any exclusion should be clearly explained and should be based on objective criteria (e.g., studies in which it was unclear that the study population was actually exposed [e.g., dry-cleaning workers]).
Weighting and classifying schemes
Subjective quality scoring (e.g., tiers, groups) should not be used in the analysis. Instead, studies should be classified in terms of their characteristics, such as studies in which exposure to trichloroethylene was well documented or based on the study’s design (e.g., cohort, case-control). These study characteristics should be examined as possible reasons for any observed heterogeneity, and meta-regression should be carried out, if deemed feasible.
Both case-control and cohort studies should be included and combined unless this introduces substantial heterogeneity into the analysis.
Tests of heterogeneity should be performed for all analyses. If heterogeneity is found to exist then a thorough search should be made to determine whether there are any explanations for the heterogeneity, such as differences in population exposures.
Fixed and random effects models should be fitted to the data. If there is no evidence for heterogeneity, then fixed models may be preferred, but it is still appropriate to report results from random effects models as well.
A sensitivity analysis should be performed in which each study is excluded from the analysis one at a time to determine whether any study significantly influences the findings. The findings from the meta-analysis should be viewed cautiously if they are highly dependent on the inclusion of one or two studies, and these studies have severe methodological limitations.