Authors' reply to Huffman and colleagues.

1Department of Health Care Policy, Harvard Medical School, Ipswich, MA 01938, USA; 2Department of Social Science, York University, Toronto, Ontario, Canada; 3Division of Biostatistics, School of Public Health Department of Statistics, University of California, Berkeley, CA, USA; 4Therapeutics Initiative, Departments of Anesthesiology, Pharmacology and Therapeutics and Medicine, University of British Columbia, Vancouver, BC, Canada