We discuss the impact of the treatment of NMC structure function data on parton distributions in the context of the NNPDF2.1 global PDF determination at NLO and NNLO. We show that the way these data are treated, and even their complete removal, has no effect on parton distributions at NLO, and at NNLO has an effect which is below one sigma. In particular, the Higgs production cross-section in the gluon fusion channel is very stable.
Bibliographical note12 pages, 8 figures. Discussion extended to NNLO, and paper accordingly rewritten altogether. Several further small improvements, including new comparison plots to SLAC and NMC data