Evaluation of a Health State Staging System Defined by Loss of Independence in Amyotrophic Lateral Sclerosis: Assessment in a Second Dataset - Hammond, E; Young, C; Chio, A; Van den Berg, L; Al-Chalabi, A; Berry, J; Brooks, B; Shefner, J; Turner, M; Williams, J; Li, X; Kerr, D (2013)