Accepted for Publication: November 22, 2012.
Author Contributions: Dr English and Ms Steen had full access to all of the data in the study and take responsibility for the integrity of the data and the accuracy of the data analysis. Study concept and design: English. Acquisition of data: Steen. Analysis and interpretation of data: Steen and English. Drafting of the manuscript: Steen and English. Critical revision of the manuscript for important intellectual content: English. Obtained funding: Steen. Administrative, technical, or material support: English. Study supervision: English.
Conflict of Interest Disclosures: None reported.
Funding/Support: Ms Steen received a stipend from the University of Pittsburgh Medical School, Dean's Summer Research Program, for her contribution.
Role of the Sponsors: The sponsors had no role in the design and conduct of the study; in the collection, analysis, and interpretation of data; or in the preparation, review, or approval of the manuscript.
Additional Contributions: We are indebted to statisticians Dan Winger, MS, and Li Wang, MS, who are supported by National Institutes of Health grants UL1 RR024153 and UL1TR000005.