This study is to determine the efficacy of momelotinib (MMB) versus best available therapy (BAT) in anemic or thrombocytopenic adults with primary myelofibrosis (PMF), or post-polycythemia vera or post-essential thrombocythemia myelofibrosis (Post-PV/ET MF) who were treated with ruxolitinib as measured by splenic response rate at Week 24 (SRR24).