  Any NLO calculation of a QCD final-state observable involves Monte Carlo
integration over a large number of events. For DIS and hadron colliders this
must usually be repeated for each new PDF set, making it impractical to
consider many `error' PDF sets, or carry out PDF fits. Here we discuss ``a
posteriori'' inclusion of PDFs, whereby the Monte Carlo run calculates a grid
(in x and Q) of cross section weights that can subsequently be combined with an
arbitrary PDF. The procedure is numerically equivalent to using an interpolated
form of the PDF. The main novelty relative to prior work is the use of
higher-order interpolation, which substantially improves the tradeoff between
accuracy and memory use. An accuracy of about 0.01% has been reached for the
single inclusive cross-section in the central rapidity region |y|<0.5 for jet
transverse momenta from 100 to 5000 GeV. This method should facilitate the
consistent inclusion of final-state data from HERA, Tevatron and LHC in PDF
fits, thus helping to increase the sensitivity of LHC to deviations from
standard Model predictions.
