I can never remember who wrote these articles, but I was fascinated by them and someday want to write a comment on them. Here they are, future-Derrill, so you can find them next time:
Autor, Katz, and Krueger (1998 QJE) for computers; DiNardo and Pischke (1997 QJE) wrote the pencils paper. See also Card and DiNardo (2002 J. Lab Ec).