“…Since this problem is known to be NP-hard ( Du & Leung, 1990 ), during the last years several approximate algorithms -heuristics and metaheuristics-have been proposed in the literature (see e.g. Karabulut, 2016;Li, Chen, Xu, & Gupta, 2015;Vallada, Ruiz, & Minella, 2008 ). However, these proposals have not been compared against themselves, or the comparison has not been carried out under the same conditions, so the state-of-the-art regarding approximate algorithms for the problem remains unclear.…”