  We discuss the uncertainty on processes computed using next-to-next-to
leading (NNLO) parton distributions (PDFs) due to the neglect of higher order
perturbative corrections in the PDF determination, in the specific case of
Higgs production in gluon fusion. By studying the behaviour of the perturbative
series for this process, we show that this uncertainty is negligible in
comparison to the theoretical uncertainty on the matrix element. We then take
this as a case study for the use of the Cacciari-Houdeau method for the
estimate of theoretical uncertainties, and show that the method provides an
effective way of treating theoretical uncertainties on the matrtix element and
the PDF on the same footing.
