To the Editor.
—The article entitled "Depressed Thyroid Indexes Associated with Occupational Exposure to Inorganic Lead," by Robins et al in the February Archives (1983;143:220-224) is misleading. The cross-sectional component of the study assumes that lead exposure preceded depression of the thyroid indexes in the foundry workers studied. No such assumption can be made in a cross-sectional study, as there is no time variable measured or implied by which such an assumption can be based. Specifically, identical results would have been obtained in this study if depression of the thyroid indexes had occurred in some way prior to the workers' exposure to lead. Without the time factor, it is not scientifically possible to assess "which came first, the chicken or the egg."