Population confidence in the health system in 15 countries: results from the first round of the People's Voice Survey by Kruk, ME, Kapoor, NR, Lewis, TP, Arsenault, C, Boutsikari, EC, Breda, J, Carai, S, Croke, K, Dayalu, R, Fink, G, Garcia, PJ, Kassa, M, Mohan, S, Moshabela, M, Nzinga, J, Oh, J, Okiro, EA, Prabhakaran, D, SteelFisher, GK, Tarricone, R, Garcia-Elorrio, E