How did they calculate IQ loss due to lead exposure specifically? https://www.pnas.org/doi/abs/10.1073/pnas.2118631119 https://www.pnas.org/doi/abs/10.1073/pna

How did they calculate IQ loss due to lead exposure specifically? https://www.pnas.org/doi/abs/10.1073/pnas.2118631119 https://www.pnas.org/doi/abs/10.1073/pnas.2118631119 I don't have access and the abstract isn't very informative in terms of sample size control group etc.