  An improved analysis method to extract quark helicity distributions in
leading order (LO) QCD from semi-inclusive double spin asymmetries in deep
inelastic scattering is presented. The method relies on the fact that
fragmentation functions, describing the fragmentation of a quark into a hadron,
have a strong dependence on the energy fraction $z$ of the observed hadron.
Hadrons with large $z$ contain more information about the struck quark. This
can be used in a weighting procedure to improve the figure of merit (= inverse
of variance). In numerical examples it is shown that one could gain 15-39\%
depending on the quark flavor and cut on $z$.
  Mathematically the problem can be described as finding an optimal solution in
terms of the figure of merit for parameters $\boldsymbol \Theta$ determined
from a system of linear equations ${\bf B}(x) {\boldsymbol \Theta} ={\bf
Y}(x)$, where the measured input vector ${\bf Y}(x)$ is given as event
distributions depending on a random variable $x$, the coefficients of the
matrix ${\bf B}(x)$ depend as well on $x$, whereas the parameter vector
$\boldsymbol\Theta$ to be determined does not.
