Centering theory is the best-known framework for theorizing about local coherence and salience; however, its claims are articulated in terms of notions which are only partially specified, such as “utterance,” “realization,” or “ranking.” A great deal of research has attempted to arrive at more detailed specifications of these parameters of the theory; as a result, the claims of centering can be instantiated in many different ways. We investigated in a systematic fashion the effect on the theory's claims of these different ways of setting the parameters. Doing this required, first of all, clarifying what the theory's claims are (one of our conclusions being that what has become known as “Constraint 1” is actually a central claim of the theory). Secondly, we had to clearly identify these parametric aspects: For example, we argue that the notion of “pronoun” used in Rule 1 should be considered a parameter. Thirdly, we had to find appropriate methods for evaluating these claims. We found that while the theory's main claim about salience and pronominalization, Rule 1—a preference for pronominalizing the backward-looking center (CB)—is verified with most instantiations, Constraint 1–a claim about (entity) coherence and CB uniqueness—is much more instantiation-dependent: It is not verified if the parameters are instantiated according to very mainstream views (“vanilla instantiation”), it holds only if indirect realization is allowed, and is violated by between 20% and 25% of utterances in our corpus even with the most favorable instantiations. We also found a trade-off between Rule 1, on the one hand, and Constraint 1 and Rule 2, on the other: Setting the parameters to minimize the violations of local coherence leads to increased violations of salience, and vice versa. Our results suggest that “entity” coherence—continuous reference to the same entities—must be supplemented at least by an account of relational coherence.

The definitions of the basic concepts, rules, and constraints of centering theory involve underspecified notions such as 'previous utterance', 'realization', and 'ranking'. We attempted to find the best way of defining each such notion among those that can be annotated reliably, and using a corpus of texts in two domains of practical interest. Our main result is that trying to reduce the number of utterances without a backward-looking center (CB) results in an increased number of cases in which some discourse entity, but not the CB, gets pronominalized, and viceversa.