Investigating sources of variability in metabolomic data in the EPIC study: the Principal Component Partial R-square (PC-PR2) method by Fages, A, Ferrari, P, Monni, S, Dossus, L, Floegel, A, Mode, N, Johansson, M, Travis, R, Bamia, C, Sanchez-Perez, M, Chiodini, P, Boshuizen, H, Chadeau-Hyam, M, Riboli, E, Jenab, M, Elena-Herrmann, B