Statistical analyses: There is no mention of the tests conducted, but data were stated to be analyzed using SAS, suggesting appropriate tests were applied. Results provided in the tables indicate that t-tests comparing IQ values between the villages (overall and by sex) were conducted, but it was not reported that heterogeneity of variance was assessed. In addition, correlations between IQ and age, family income, and parents’ education level were tested with Pearson's correlation. There is no evidence that a test for trend was conducted to evaluate the stated “significant inverse concentration-response relationship between the fluoride level in drinking water and the IQ of children.”

A potential concern raised by the NASEM (2020) committee’s review was the lack of accounting for relationships in exposure between persons from the same village. Given only two villages were included and the analyses consisted of village-level comparisons (no use of individual-level covariate data), it is likely that the standard error of the difference in mean IQ between fluoride in water exposure groups will be biased, making differences appear stronger than they actually are. Without controlling for village effects and given the large differences in fluoride concentrations and IQ levels between villages, the apparent dose-response relationship could be due to a village effect in addition to a fluoride effect. However, a dose-response relationship is apparent within the “exposed” village, diminishing the concern for a village-only effect and likely minimizing the impact on the effect estimates.

Other potential concerns: None identified.

Xiang 2003a

Risk of bias

Epidemiological study populations

HAWC