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We study the uncertainties in parton distributions, determined in global fitsto deep inelastic and related hard scattering data, due to so-calledtheoretical errors. Amongst these, we include potential errors due to thechange of perturbative order (NLO to NNLO), ln(1/x) and ln(1-x) effects,absorptive corrections and higher-twist contributions. We investigate theseuncertainties both by including explicit corrections to our standard globalanalysis and by examining the sensitivity to changes of the x,Q^2,W^2 cuts onthe data that are fitted. In this way we expose those kinematic regions wherethe conventional DGLAP description is inadequate. As a consequence we obtain aset of NLO, and of NNLO, conservative partons where the data are fullyconsistent with DGLAP evolution, but over a restricted kinematic domain. Wealso examine the potential effects of such issues as the choice of inputparameterization, heavy target corrections, assumptions about the strange quarksea and isospin violation. Hence we are able to compare the theoretical errorswith those uncertainties due to errors on the experimental measurements, whichwe studied previously. We use W and Higgs boson production at the Tevatron andthe LHC as explicit examples of the uncertainties arising from partondistributions. For many observables the theoretical error is dominant, but forthe cross section for W production at the Tevatron both the theoretical andexperimental uncertainties are small, and hence the NNLO prediction may serveas a valuable luminosity monitor.

Uncertainties of predictions from parton distributions II: theoretical errors