1 Special Electives of Comp.Linguistics: Processing Anaphoric Expressions Eleni Miltsakaki AUTH Fall 2005-Lecture 4

2 Main points on Centering What is it about? –A theory that relates focus of attention, choice of referring expression and perceived coherence of utterances within a discourse segment –Centering captures the difference in perceived text coherence due to attention structure Outline of the Centering model –Discourse  segments  utterances –Set of forward-looking centers, {Cf1, Cf2…} –Preferred center, Cp –Backward-looking center, Cb –Cf ranking (Sub>Obj>Other) –Ordering transitions Centering rules –Pronoun rule –Center transition rule

3 Centering transitions

4 Applying Centering to garden path 1.Terry really goofs sometimes. 2.Yesterday was a beautiful day and he was excited about trying out his new sailboat. 3.He wanted Tony to join him on a sailing expedition. 4.He called him at 6am. 5.He was sick and furious at being woken up so early

Historical development CT CT Complexity of inferencing Almost Monadic PC Attention Reference Coherence Alternatives Functional Linear Semantic Joshi&Kuhn 1979 Joshi&Weinstein 1981 G&S 1986 GJ&W 1986 G,J& W 1995 Strube&Kuhn 1996 Walker 1996 Stevenson et al 2000

6 Centering-based anaphora resolution The BFP algorithm (Brennan, Friedman and Pollard, 1987) Functional centering (Strube and Hahn 1996, 1997) The S-list algorithm (Strube 1998) RAFT/RAPR algorithm (Suri and McCoy 1994, Suri, McCoy and DeCristofaro 1999)

7 The BFP algorithm Assuming the Centering model, The BFP algorithm consists of three basic steps –Generate possible Cb-Cf combinations –Filter by constraints (e.g., contra-indexing, centering rules, etc) –Rank by transition orderings

8 Applying the BFP algorithm 1. Brennan drives an Alfa Romeo 2. She drives too fast. 3. Friedman races her on weekends. 4. She often beats her.

9 Centering analysis a la BFP

10 Shortcomings of the BFP algorithm Resolution error when Cb(Ui-1) is different from Cp(Ui-1) –Ellen-i saw Mary-j at school –Mary-j didn’t talk to her-i –She-j took her-j friends and walked away Ambiguity when Ui-1 is discourse initial –John gave a lot of his property to George –His current salary exceeded the average salary by a lot.

11 Functional Centering Cf ranking is based on functional information status Danes (1974) trichotomy –Given information  Cb –Theme  Cp –New information (rheme)  Elements of Ui not contained in Ui-1 Claim: Functional ranking is universal

12 More detailed Cf ranking Based on Prince’s familiarity scale (1981) –Hearer old: Evoked (E), Unused (U) –Hearer new: Brand-new (BN), Anchored brand new (Ba), Inferrable (I), Containing Inferrable (Ic)

13 Rambow’s diagnostic See relevant section in Miltsakaki 2002

14 Functional Centering analysis

15 The S-list algorithm Basics –No Cbs! –S-list: the list of salient discourse entities –S-list ranking criteria –Hearer-old > hearer-new The algorithm –The S-list is generated incrementally and is updated every time an anaphoric element is resolved –Resolution is done by looking-up in the S-list, starting from the highest ranked –Discourse entities from the S-list that are not realized in the current utterance are removed