  We discuss the causes which can limit the accuracy of the predictions based
on the conventional PDFs when including in global parton analyses the data at
moderate scales $\mu$. The first is the existence of power corrections ${\cal
O} (Q_0^2/\mu^2)$ due to the double counting of contributions arising from the
region below the input scale $Q_0$. The second concerns the possible inclusion
of the BFKL re-summation of the $(\alpha_s\ln (1/x))^n$ terms. The third is the
treatment of the heavy-quark thresholds. We show how to include the heavy-quark
masses ($m_h$ with $h=c,b,t$) in DGLAP evolution which provides the correct
{\it smooth} behaviour through the threshold regions and how to subtract the
low parton virtuality $|k^2|<Q^2_0$ contributions from the DIS and Drell-Yan
NLO coefficient functions in order to avoid the double counting.
