The effect on PDFs and [Formula: see text] due to changes in flavour scheme and higher twist contributions.

I consider the effect on MSTW partons distribution functions (PDFs) due to changes in the choices of theoretical procedure used in the fit. I first consider using the 3-flavour fixed flavour number scheme instead of the standard general mass variable flavour number scheme used in the MSTW analysis. This results in the light quarks increasing at all relatively small [Formula: see text] values, the gluon distribution becoming smaller at high values of [Formula: see text] and larger at small [Formula: see text], the preferred value of the coupling constant [Formula: see text] falling, particularly at NNLO, and the fit quality deteriorates. I also consider lowering the kinematic cut on [Formula: see text] for DIS data and simultaneously introducing higher twist terms which are fit to data. This results in much smaller effects on both PDFs and [Formula: see text] than the scheme change, except for quarks at very high [Formula: see text]. I show that the structure function one obtains from a fixed input set of PDFs using the fixed flavour scheme and variable flavour scheme differ significantly for [Formula: see text] at high [Formula: see text], and that this is due to the fact that in the fixed flavour scheme there is a slow convergence of large logarithmic terms of the form [Formula: see text] relevant for this regime. I conclude that some of the most significant differences in PDF sets are largely due to the choice of flavour scheme used.