Previous experimental studies of various correlation functions in angular i
ntervals and their comparison with QCD calculations are summarized. A good
description is only obtained for a globally normalized function ((r) over b
ar). The hypothesis of Local Parton Hadron Duality (LPHD) seems to be well
fulfilled in this case. For all differentially normalized functions, howeve
r, several disagreements are observed and the validity of LPHD is in questi
on. It is argued (using Monte Carlo calculations) that LPHD could still be
valid also in this case - after improving the perturbative QCD calculations
. Only after the inclusion of full energy-momentum conservation in the calc
ulations one will have a better handle to estimate, how far non-perturbativ
e effects inc spoiling the agreement with the data.