  The modified perturbation theory (MPT), based on direct expansion of
probabilities instead of amplitudes, allows one to avoid divergences in the
phase-space integrals resulting from production and decay of unstable
particles. In the present paper the range of applicability of MPT is determined
numerically in the case of the process $e^+e^- \to (\gamma,Z) \to t\bar t \to
W^{+}b:W^{-}\bar b$. It is shown that with the complete expansion in powers of
the coupling constant (without Dyson resummation) MPT operates best at the
energies located near the maximum of the cross-section and slightly above the
maximum. In this region the MPT expansion within the next-to-leading order
considerably exceeds in accuracy well-known DPA approach.
