Strengthening the reporting of observational studies in epidemiology (STROBE): explanation and elaboration - Vandenbroucke, J P; Elm von, E; Altman, D G; Gøtzsche, P C; Mulrow, C D; Pocock, S J; Poole, C; Schlesselman, J J; Egger, M; STROBE Initiative (Epidemiology, 2007)