*This research was supported by a grant from the Public Health Service. We are indebted to Joseph Newhouse for helpful comments. The opinions, and any remaining errors, are those of the authors and do not necessarily reflect the views of the Public Health Service.
Notes
*This research was supported by a grant from the Public Health Service. We are indebted to Joseph Newhouse for helpful comments. The opinions, and any remaining errors, are those of the authors and do not necessarily reflect the views of the Public Health Service.