Stephan Schiffman, direktur D.E.I. Management Group, adalah penulis buku-buku laris tentang penjualan, termasuk "Cold Calling Techniques (That Really Works)" dan "The 250 Sales Questions to Close the Deal". Klien-kliennya antara lain Aetna, AT&T, Blue Cross/Blue Shield, Boise Office Solutions, Chevron Texaco, Cox Communications, EMC, ExxonMobil, Federal Express, IBM, Merrill Lynch, Motorola, New York Times, Sony, dan Waste Management.