  The privacy amplification term, of which the lower bound needs to be
estimated with the decoy-state method, plays a positive role in the secure key
rate formula for decoy-state quantum key distribution. In previous work, the
yield and the bit error rate of single-photon state are estimated separately to
gain this lower bound. In this work, we for the first time take the privacy
amplification term as a whole to consider this lower bound. The mathematical
description for the correlation between the yield and the bit error rate of
single-photon state is given with just two unknown variables. Based on this, we
obtain the global estimation of this lower bound for both BB84 protocol and
measurement-device-independent protocol. The results of numerical simulation
show that the global estimation can significantly improve the performance of
quantum key distribution.
