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Crf

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Crf

  1. 1. BIONLP09 and CRFs Farzaneh Sarafraz 18 February 2009    
  2. 2. BioNLP'09 Event rather than entity  Most entities are given  3 tasks  − Event detection and characterization − Event argument recognition − Negations and speculations    
  3. 3. Example quot;I kappa B/MAD­3 masks the nuclear localization  signal of NF­kappa B p65 and requires the  transactivation domain to inhibit NF­kappa B  p65 DNA binding. quot; Event: negative regulation Trigger: masks Theme1: the first p65 Cause: MAD­3 Site: nuclear localization signal    
  4. 4. Example quot;In contrast, NF­kappa B p50 alone fails to  stimulate kappa B­directed transcription, and  based on prior in vitro studies, is not  directly regulated by I kappa B. quot; Event: regulation Theme1: this p50 Trigger: regulated Negation: true for this event Speculation: none    
  5. 5. HMM and MEMM (X1, X2, ...) Observations (Y1, Y2, ...) labels p(Xi , Yi)   X  ranges over observation sequence  Y ranges over and label sequence Requires independence assumption  i.e. each item is labelled independently    
  6. 6. Conditional Random Field p(Y |X)  Y: label sequence X: observation sequence Maximise p     
  7. 7. MMEM Label Bias Problem Probability given the current state  − Transitions leaving a state compete against each other  not all states  − Per­state normalization − Probability bias towards states with few transitions − Demonstrated experimentall    
  8. 8. Label Bias Example Training data:  − A B C D − A B D D − A B C E − A B D C Model says:  − C > D 50% − C > E 50% Why predict E when D is much more common?    
  9. 9. CRF Solution Model probability of transitions and probability   of states CRFs  − Models probability of transition between states − Probability is conditional on current observation − Not normalised − Considers many quot;featuresquot; of observations    
  10. 10. Features quot;edge featuresquot; as well as quot;vertex featuresquot;  − Word is capitalized − Word ends in quot;­ingquot; − Label is quot;proper nounquot; Features are important!     
  11. 11. End.    

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