Originally published in Health Services Research, Volume 46, Issue 1, Part 1 (February 2011)
The article was originally published without its Joint Acknowledgment/Disclosure Statement. The statement is printed below:
Joint Acknowledgment/Disclosure Statement: Sophia Rabe-Hesketh and Anders Skrondal greatly influenced this paper through their writings on multi-level modeling. Alice Mroszczyk and Phil Mello of the Massachusetts Dept of Public Health provided invaluable administrative assistance. Thanks to the editors, two anonymous referees, Rich Embrey, Bob Kaplan, Michael Ong, Eric Peterson, Kevin Schulman and Todd Seto for formative discussions and comments. Errors remain my own.