Challenges in applying the GRADE approach in public health guidelines and systematic reviews : a concept article from the GRADE Public Health Group - Hilton Boon, Michele (MRC/CSO Social and Public Health Sciences Unit, University of Glasgow) ; Thomson, Hilary (MRC/CSO Social and Public Health Sciences Unit, University of Glasgow) ; Shaw, Beth (Center for Evidence-based Policy, Oregon Health & Science University) ; Akl, Elie A. (Department of Internal Medicine, American University of Beirut) ; Lhachimi, Stefan K. (Health Sciences Bremen, University of Bremen) ; Lopez-Alcalde, Jesus (Universitat Autònoma de Barcelona. Departament de Pediatria, d'Obstetrícia i Ginecologia i de Medicina Preventiva) ; Klugar, Miloslav (Faculty of Medicine, Czech National Centre for Evidence-Based Healthcare and Knowledge Translation) ; Choi, Leslie (The Department of Vector Biology, Partnership for Increasing the Impact of Vector Control, Liverpool School of Tropical Medicine) ; Saz-Parkinson, Zuleika (European Commission, Joint Research Centre, Ispra, Italy) ; Mustafa, Reem A. (Departments of Medicine and Biomedical & Health Informatics, University of Missouri-Kansas City) ; Langendam, Miranda W. (Amsterdam Universitair Medische Centra (Amsterdam, Països Baixos)) ; Crane, Olivia (National Institute for Health and Care Excellence (NICE)) ; Morgan, Rebecca L. (Department of Health Research Methods, Evidence, and Impact, McMaster University) ; Rehfuess, Eva (Institute for Medical Informatics, Biometry and Epidemiology, Pettenkofer School of Public Health, LMU Munich) ; Johnston, Bradley C. (Department of Nutrition, Texas A&M University) ; Chong, Lee Yee (Cochrane Public Health and Health Systems Network, University of Oxford) ; Guyatt, Gordon H. (Department of Health Research Methods, Evidence, and Impact, McMaster University, Canada) ; Schünemann, Holger (Michael G DeGroote Cochrane Canada) ; Katikireddi, Srinivasa Vittal (MRC/CSO Social and Public Health Sciences Unit, University of Glasgow)