1. Center for Clinical Epidemiology and Biostatistics, and Department of Biostatistics & Epidemiology, University of Pennsylvania School of Medicine, 826 Blockley Hall, 423 Guardian Drive, Philadelphia, PA 19104-6021, USA.
2. Department of Neurology, University of Alabama at Birmingham, JT 1235, 619 South 19th Street, Birmingham, AL 35249, USA.
3. Center for Clinical Epidemiology and Biostatistics, and Department of Biostatistics & Epidemiology and Department of Medicine, University of Pennsylvania School of Medicine, 717 Blockley Hall, 423 Guardian Drive, Philadelphia, PA 19104–6021, USA.