On the basis of the dynamical interpretation of Monte Carlo simulations, we discuss the relation of the equilibrium relaxation time, the susceptibility and the statistical error. We introduce a new quantity called {\it the statistical dependence time} $��_{dep}$, which gives the reduction factor for the statistical degree of freedom due to the dynamical correlations between the data. A new method is proposed for calculating equilibrium relaxation time using $��_{dep}$, the method which does not require knowledge of any time-displaced correlation function. We apply this method to the critical dynamics of Ising models in two and three dimensions, and estimate the dynamical critical exponent $z$ precisely. Systematic errors in response functions due to short simulations are also discussed from the viewpoint of the statistical dependence.

kikuchi@godzilla.kek.jp; LaTeX, 9 pages, 6 figures; to appear in "Computer Simulations in Condensed Matter Physics VII", ed. D.P. Landau, K.K. Mon and H.B. Sch\"uttler (Springer)