We apply our systematic NLO small x resummation of singlet splitting
functions to the scaling violations of structure functions and compare the
results with data. We develop various theoretical tools which are needed in
order to relate resummed parton distributions to measurable structure
functions, and we present results from a variety of fits to HERA data for the
structure functions F_2 and F_L using the resummation. The behaviour of the
singlet splitting functions at small x and fixed Q^2 is effectively
parametrized as x^{-lambda}. We find that, for lambda small or negative, the
resummed description of scaling violations may be phenomenologically as good as
or even better than the standard next-to-leading order treatment. However, the
best fit gluon density and value of alpha_s can be significantly modified by
the resummation.Comment: 40 pages, 15 figures. Final version, to be published in Nucl. Phys.
B. Typos corrected in eq. 4.5 and eq. 4.20 and in caption to fig.
