For instance, in some Centering papers, entities are ranked with respect to their grammatical function (Brennan, Friedman, and Pollard 1987;Walker, Iida, and Cote 1994;Grosz, Joshi, and Weinstein 1995), and in others with respect to their position in Prince's (1981) givenness hierarchy (Strube and Hahn 1999) or their thematic role (Sidner 1979).
two "instantiations" of the same theory make different predictions for the same input.