Correction: Predictors of opioid efficacy in patients with chronic pain: A prospective multicenter observational cohort study

by Kasper Grosen, Anne E. Olesen, Mikkel Gram, Torsten Jonsson, Michael Kamp-Jensen, Trine Andresen, Christian Nielsen, Gorazd Pozlep, Mogens Pfeiffer-Jensen, Bart Morlion, Asbjørn M. Drewes