Description:
In Reply.— Dr Needles raises several important points about our study. He questions the presence of a dose-effect relationship between blood lead level at age 24 months and intellectual functioning at age 10, as well as our assertion that a threshold for this relationship was not apparent. His skepticism is understandable. Our conclusions were based on three observations, not all of which we were able to include in our paper. 1) Examination of the partial regression residual plot (IQ adjusted for all covariates and blood lead level adjusted for all covariates) indicated symmetrical distribution of residuals about the regression line and similarity in the residual variances over the blood lead range.