04
4 What’s a paraphrase of… Paraphrase
System “is on good terms with” “is friendly with”
“is a friend of”
… Query expansion
Document summarization
Natural language generation
Question answering
etc. Applications<br>
05
5 What’s a paraphrase of…<br>
06
6 Bilingual Parallel Corpus …the cost dynamic is under control… …die kostenentwicklung unter kontrolle… …keep the cost in check… …die kosten unter kontrolle… … … Phrase Table<br>
07
BCB system [Bannard & Callison-Burch, ACL’05]
P(E2|E1) ¼C G P(E2|G) P(G|E1)
SBP system [Callison-Burch, EMNLP’08]
P(E2|E1) ¼C G P(E2|G,syn(E1)) p(G|E1, syn(E1)) 7 State of the Art<br>
08
8 E1 E2 G1 F2 E3 E4 (in check) (under control) G2 G3 (unter kontrolle) F1 Graphical View<br>
09
9 Graphical View Path lengths > 2 Add nodes to represent domain knowledge G1 F2 G2 G3 F1 E1 E2 E3 E4<br>
10
Motivation
Background
Hitting Time Paraphraser
Experiments
Future Work 10 Overview<br>
11
A A Random Walk Begin at node A
Randomly pick neighbor n E F D B C 11<br>
12
Random Walk Begin at node A
Randomly pick neighbor n
Move to node n E F D A 2 B C 12<br>
13
Random Walk Begin at node A
Randomly pick neighbor n
Move to node n
Repeat E F D A B 2 C 13<br>
14
Expected number of steps starting from node i before node j is visited for first time
Smaller hitting time → closer to start node i
Truncated Hitting Time [Sarkar & Moore, UAI’07]
Random walks are limited to T steps
Computed efficiently & with high probability by sampling random walks [Sarkar, Moore & Prakash ICML’08] 14 Hitting Time from node i to j<br>
15
Finding Truncated Hitting Time By Sampling E F D 1 B C A A T=5 15<br>
16
Finding Truncated Hitting Time By Sampling E F 4 A B C D A D T=5 16<br>
17
Finding Truncated Hitting Time By Sampling 5 F D A B C E A D E T=5 17<br>
18
Finding Truncated Hitting Time By Sampling E F 4 A B C D A D E D T=5 18<br>
19
Finding Truncated Hitting Time By Sampling E 6 D A B C F A D E D F T=5 19<br>
20
Finding Truncated Hitting Time By Sampling 5 F D A B C E A D E D F E T=5 20<br>
21
Finding Truncated Hitting Time By Sampling A D E D F E T=5 E F D A B C hAD=1 hAE=2 hAF=4 hAA=0 hAB=5 hAC=5 21<br>
22
Motivation
Background
Hitting Time Paraphraser
Experiments
Future Work 22 Overview<br>
23
23 Hitting Time Paraphraser
(HTP) Phrase Tables English-German
English-French
German-French
etc. Phrase Paraphrases<br>
24
24 Graph Construction<br>
25
25 Graph Construction<br>
26
BFS from query phrase up to depth d or up to max. number n of nodes
d = 6, n = 50,000 26 … … … … … … … … … Graph Construction<br>
27
27 Graph Construction … … … … … … … … …<br>
28
28 Graph Construction … … … … … … … … … 0.6<br>
29
29 Graph Construction … … … … … … … … … 0.5 0.5<br>
30
Run m truncated random walks to estimate truncated hitting time of each node
T = 10, m = 1,000,000
Prune nodes with hitting times = T Estimate Trunc. Hitting Times<br>
31
31 Add Ngram Nodes “achieve the goal” “achieve the aim” “reach the objective”<br>
32
32 Add “Syntax” Nodes “whose goal is” “the aim is” “the objective is” “what goal” start with article end with be start with
interrogatives<br>
33
33 Add Not-Substring-Of Nodes “reach the” “reach the aim” “reach the objective” “objective” not-substring-of<br>
34
34 Feature Nodes ngram nodes “syntax” nodes not-substring nodes phrase nodes<br>
35
Run m truncated random walks again
Rank paraphrases in increasing order of hitting times 35 Re-estimate Truncated Hitting Times<br>
36
Motivation
Background
Hitting Time Paraphraser
Experiments
Future Work 36 Overview<br>
37
Europarl dataset [Koehn, MT-Summit’05]
Use 6 of 11 languages: English, Danish, German, Spanish, Finnish, Dutch
About a million sentences per language
English−Foreign phrasal alignments by giza++ [Callison-Burch, EMNLP’08]
Foreign−Foreign phrasal alignments by MSR aligner 37 Data<br>
38
SBP system [Callison-Burch, EMNLP’08]
HTP with no feature node
HTP with bipartite graph 38 Comparison Systems<br>
39
NIST dataset
4 English translations per Chinese sentence
33,216 English translations
Randomly selected 100 English phrases
From 1-4grams in both NIST & Europarl datasets
Exclude stop words, numbers, phrases containing periods and commas 39 Evaluation Methodology<br>
40
For each phrase, randomly select a sentence from NIST dataset containing it
Substituted top 1 to 10 paraphrases for phrase 40 Methodology<br>
41
Manually evaluated resulting sentences
0: Clearly wrong; grammatically incorrect or does
not preserve meaning
1: Minor grammatical errors (e.g., subject-verb
disagreement; wrong tenses, etc.),
or meaning largely preserved but not completely
2: Totally correct; grammatically correct and
meaning is preserved
Correct: 1 and 2; Wrong: 0
Two evaluators; Kappa = 0.62 (substantial agree.) 41 Methodology<br>
43
43 HTP vs. SBP 373
paraphrases per system<br>
44
44 HTP vs. SBP 483
paraphrases 0.54<br>
45
45 HTP vs. SBP 0.53 0.50<br>
46
46 HTP vs. SBP 975
paraphrases 0.32 373
paraphrases 492
paraphrases 0.43 420 correct
paraphrases 145 correct
paraphrases<br>
48
Motivation
Background
Hitting Time Paraphraser
Experiments
Future Work 48 Overview<br>
49
Apply HTP to languages other than English
Evaluate HTP impact on applications
e.g., improve performance of resource-sparse machine translation systems
Add more features
etc. 49 Future Work<br>
50
HTP: a paraphrase system based on random walks
Good paraphrases have smaller hitting times
General graph
Path length > 2
Incorporate domain knowledge
HTP outperforms state-of-the-art 50 Conclusion<br>