Answer Problem 6.67 for full-scale IQ?

We have previously described Data Set LEAD.DAT (at www.cengagebrain.com), in which children were classified according to blood-lead level in 1972 and 1973 by the variable lead_grp, where 1 = blood-lead level < 40 μg/100 mL in both 1972 and 1973, 2 = blood-lead level ≥ 40 μg/100 mL in 1973, 3 = blood-lead level ≥ 40 μg/100 mL in 1972 but < 40 μg/100 mL in 1973.