1. Division of Pediatric Hematology Oncology, Department of Pediatrics University of Alabama at Birmingham Birmingham Alabama USA
2. Jimmy Everest Section of Pediatric Hematology Oncology, Department of Pediatrics, University of Oklahoma Health Sciences Center Oklahoma City Oklahoma USA
3. Department of Epidemiology & Biostatistics University of Oklahoma Health Sciences Center Oklahoma City Oklahoma USA
4. Department of Economics Michigan State University East Lansing Michigan USA