Statistical analyses inadequate controls

A decrease in erythrocyte 6-aminolevulinate dehydratase activity was observed in rats exposed to 200 but not 50 ppm tetrachloroethylene for 4 weeks (Soni et al. 1990). It is not clear if exposure was intermittent or continuous. Rats exposed to 230 or 470 ppm tetrachloroethylene for up to 160 days had splenic congestion and increased hemosiderin deposits (Carpenter 1937). Study limitations include the use of sick animals (parasites, pneumonia), nonstandard study protocols, rats of undefined strain, and inadequate controls. A transient increase in reticulocytes was observed in mice exposed to tetrachloroethylene at 135 and 270 ppm during the first few weeks of an 11.5-week study (Seidel et al. 1992). Microscopic examination of bone marrow revealed no effect on pluripotent stem cells and only a small reduction in erythroid committed cells. Because of a lack of statistical analysis, NOAELs and LOAELs were not clearly identified in the Seidel et al. (1992) study. 

Shafer et al. (2005) reported that there is no information on age-dependent toxicity for most pyrethroids. Their review summarizes the results of 22 studies performed on the developmental neurotoxicity of pyrethroids (strengths and limitations of studies). Many of the studies suffer from inadequate study design, problematic statistical analysis, use of formulated products (e.g., unknown isomer mixtures), and/or inadequate controls. 

Epidemiologic analysis of data in man often fails to yield conclusive results because the size of the study group is too small (statistical power is inadequate to detect a difference), exposure data are lacking, concomitant or prior exposures to other known or suspected carcinogens may interact with the compound(s) in question and confound the analysis, or a proper control group cannot be identified. 

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