In this section we present the main features of the framework underlying the
global QCD analysis of parton distribution functions.
%
First of all, we discuss how PDFs are parametrized at
the reference scale, and review the theoretical constraints
that should be imposed on this parametrization such as
the momentum and valence sum rules and positivity.
%
Then we discuss how to quantify the agreement of data and
theory, and review various methods used in global analysis
for minimization of the figure of merit $\chi^2$.
%
We  also review here the different strategies available to estimate
and propagate PDF uncertainties, with emphasis on the three
most important ones: the Hessian, the Monte Carlo, and
the Lagrange multiplier methods.

Other related topics that are discussed in this section
include how to combine individual
PDF sets within a single set, the treatment
of theoretical parametric uncertainties,
the use of approximate methods
to estimate the impact of new experiments without redoing the PDF fit,
as well as the public delivery of PDFs by means of the
{\tt LHAPDF6} interface.

\subsection{PDF parametrization}\label{sec:fitmeth.PDFpara}
\input{4-fitting/subsec-parametrization}

\subsection{Fit quality and minimization strategies}\label{sec:fitmeth.min}
\input{4-fitting/subsec-chi2}

\subsection{PDF uncertainties}
\label{sec:pdfuncertainties}
\input{4-fitting/subsec-uncertainties}

\subsection{Combined and reduced PDF sets}
\label{sec:fitmeth.combined}
\input{4-fitting/subsec-combined}

\subsection{Treatment of theoretical parametric uncertainties}\label{sec:fitmeth.theoryunc}
\input{4-fitting/subsec-paramunc}

\subsection{Approximate methods}\label{sec:fitmeth.approx}
\input{4-fitting/subsec-reweighting}

\subsection{Public delivery: {\tt LHAPDF}}\label{subsec:fitmeth.delivery}
\input{4-fitting/subsec-delivery}

