The Ellis-Jaffe Sum Rule: The Estimates of the Next-to-Next-to- Leading-Order QCD Corrections

The procedure of the estimates of the higher-order perturbative QCD corrections to the physical quantities is generalized to the case when the quantities under consideration obey the renormalization group equationts with the corresponding anomalous dimension function. This procedure is used to estimate the $\alpha_s^3$-corrections to the singlet part of the Ellis-Jaffe sum rule for $f=3$ numbers of flavours.