  By the generator of the UrQMD model, event statistics for the products of
kurtosis ($\kappa$) and variance ($\sigma^2$) of net-proton and net-charge
multiplicity distributions are carefully studied. It is shown that the
statistics at RHIC/BES below $\sqrt {s_{NN}} < 19.6$ GeV are not sufficient for
using the method of Centrality Bin Width Correction (CBWC). Corresponding
results are systematically underestimated. A way to improve the CBWC method is
proposed. It can remove the statistics dependence of the data and reduce the
initial size fluctuation as well.
