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

2. 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. 3 Centering transitions

4. 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

5. Historical development CT 1977-1981CT 1986-1995 Complexity of inferencing Almost Monadic PC Attention Reference Coherence Alternatives 1996-2000 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. 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. 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. 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. 9 Centering analysis a la BFP

10. 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. 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. 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. 13 Rambow’s diagnostic See relevant section in Miltsakaki 2002

14. 14 Functional Centering analysis

15. 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