On the Determination of Spin-Dependent Parton Distributions in   Semi-Inclusive Deep Inelastic Scattering
  New polarized fragmentation functions are introduced and justified, in addition to those conventional ones assumed to be independent of the helicity of the parent parton. It is demonstrated that due to our present ignorance concerning these new parton-spin dependent leading-twist fragmentation functions, it is impossible to utilize current experiments on spin-dependent semi-inclusive deep inelastic lepton nucleon scattering to disentangle the separate polarized parton distributions. 
M. Gl\"uck and E. Reya (Dortmund U.)