  We report a simple method to accurately determine the threshold and the
exponent $\nu$ of the Bak-Sneppen model and also investigate the BS
universality class. For the random-neighbor version of the BS model, we find
the threshold $x^*=0.33332(3)$, in agreement with the exact result $x^*=1/3$
given by mean-field theory. For the one-dimensional original model, we find
$x^*=0.6672(2)$ in good agreement with the results reported in the literature;
for the anisotropic BS model we obtain $x^*=0.7240(1)$. We study the finite
size effect $x^*(L)-x^*(L \to \infty) \propto L^{-\nu}$, observed in a system
with $L$ sites, and find $\nu = 1.00(1)$ for the random-neighbor version, $\nu
= 1.40(1)$ for the original model, and $\nu=1.58(1)$ for the anisotropic case.
Finally, we discuss the effect of defining the extremal site as the one which
minimizes a general function $f(x)$, instead of simply $f(x)=x$ as in the
original updating rule. We emphasize that models with extremal dynamics have
singular stationary probability distributions $p(x)$. Our simulations indicate
the existence of two symmetry-based universality classes.
