  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.
