Remarks on the reweighting method in the chemical potential direction

We comment on the reweighting method in the chemical potential $(\mu_{\rm q})$ direction. We study the fluctuation of the reweighting factor during Monte-Carlo steps. We find that it is the absolute value of the reweighting factor that mainly contributes to the shift of the phase transition line $(\beta_c)$ by the presence of $\mu_{\rm q}$. The phase fluctuation is a cause of the sign problem, but the effect on $\beta_c$ seems to be small. We also discuss $\beta_c$ for Iso-vector chemical potential and $\beta_c$ determined from simulations with imaginary chemical potential.