  A new classification of sandpile models into universality classes is
presented. On the basis of extensive numerical simulations, in which we measure
an extended set of exponents, the Manna two state model [S. S. Manna, J. Phys.
A 24, L363 (1991)] is found to belong to a universality class of random
neighbor models which is distinct from the universality class of the original
model of Bak, Tang and Wiesenfeld [P. Bak, C. Tang and K. Wiensenfeld, Phys.
Rev. Lett. 59, 381 (1987)]. Directed models are found to belong to a
universality class which includes the directed model introduced and solved by
Dhar
