Errors in the preprint (NOT in the final printed version): p.211, abstract: Replace "R&D work" by "and R&D work". p.211, author address: Replace "2101 SSH" by "3053 Kemper". p.216, Table 2, Computer Science column: Currently the number 1,725 is displayed twice. The first instance (i.e. the regression coefficient for age) should be 4,389. In the second instance, remove the parentheses.