Presentation on theme: "Open-Source Implementation of Document Structuring Algorithm for NLTK Nicholas FitzGerald."— Presentation transcript:
Open-Source Implementation of Document Structuring Algorithm for NLTK Nicholas FitzGerald
Natural Language Generation Generate coherent text outputs to express information Express the right information Express information in the right order
NLG Tasks 1. Document Structuring - most important and relevant information selected from knowledge base (Content Determination), then ordered and structured in such a way as to maximize coherence and informativeness (Text Planning) 2.Micro-Planning – specifics of word selection, referring expressions, and the finalization of ordering are determined 3.Realization – internal representations of the above decisions are realized in actual text output
Document Structuring Given a set of information to be expressed, determine the order and grouping of this information Texts cannot be simply a random bag of sentences Order of message presentation has significant effect on meaning [Hovy 1993]: One way: 1 - “Maria was diagnosed with cancer some months ago.” 2 - “Maria and Zurab had a fight last night.” 3 - “She was found dead this morning.” Vs. 1 - “Maria was diagnosed with cancer some months ago.” 2 - “She was found dead this morning.” 3 - “Maria and Zurab had a fight last night.”
Document Structuring Ordering also effects coherence: “John was hungry. John went to the store. He bought some bread to make a sandwich.” “John bought some bread to make a sandwich. He went to the store. John was hungry.”
Discourse relations Relationship between a message or group of messages Elaboration(m1,m2) I love jazz music(m1). My favourite album is Oscar Peterson's “Night Train” (m2). Contrast(m1, m3) I love jazz music (m1). However, my favourite album is The Beatles' “White Album” (m3). Cue word - However
Rhetorical Structure Theory Mann and Thompson 1988 A text is coherent by virtue of relationships that hold between messages in the text A small number of relations (~25) can explain relationships between messages in a wide range of text
Project Proposal Implement these general algorithms for inclusion in NLTK Provide a sample Data Set and DR schema for testing and illustration based on hypothetical WeatherExplainer from [Reiter and Dale 2000] Experiment utilizing these new tools as part of Abstractive Summarization System for Evaluative Statement Summarization (ASSESS)
Implementation 1: Schemas Top-Down Approach Output document structure is predictable and stereotyped Schemas are patterns of expansion, similar to CFG Ie: CompareAndContrast → DescribeRelationship CompareProperties. CompareProperties → CompareProperty CompareProperties. CompareProperties →. “John is much bigger than Kate (DR). He is five inches taller (CP) and weighs almost twice as much (CP).” Specify rules for choosing if multiple expansions exist
Top-Down Problems Hypothesis-Driven Content selection done “on-line” Not easily pipelined Therefore, Bottom-Up used
Implementation 2: Bottom-Up Output document structure is not predictable POOL = messages to be expressed while( size(pool) > 1)): find all pairs of elements in pool which can be joined by a DR assign a desirability score to each potential DR find pair E i and E j with highest score and combine with E k remove E i and E j from POOL, replace with E k end while
Implementation Used nltk.featstruct for Messages and DocPlans A mapping from feature identifiers to feature values, where each feature value is either a basic value (such as a string or an integer), or a nested feature structure. TotalRainfallMsg period year 1996 month 06 attribute type 'RelativeVariation' magnitude unit 'inches' number 4 direction '+' [ *msgType* = 'TotalRainfallMsg' ] [ ] [ [ direction = '+' ] ] [ [ ] ] [ attribute = [ magnitude = [ number = 4 ] ] ] [ [ [ unit = 'inches' ] ] ] [ [ ] ] [ [ type = 'RelativeVariation' ] ] [ ] [ period = [ month = 6 ] ] [ [ year = 1996 ] ]
Implementation nltk.featstruct.FeatStruct unify(other): Unify fstruct1 with fstruct2, and return the resulting feature structure. This unified feature structure is the minimal feature structure that: contains all feature value assignments from both fstruct1 and fstruct2. preserves all reentrance properties of fstruct1 and fstruct2. If no such feature structure exists (because fstruct1 and fstruct2 specify incompatible values for some feature), then unification fails, and unify returns None.
Unification TotalRainfallMsg period year 1996 Month 06 attribute type 'RelativeVariation' direction '+' + TotalRainfallMsg period year 1996 month 06 attribute type 'RelativeVariation' magnitude unit 'inches' number 4 = TotalRainfallMsg period year 1996 month 06 attribute type 'RelativeVariation' magnitude unit 'inches' number 4 direction '+'
Implementation nltk.featstruct.FeatStruct subsumes(other): True if self subsumes other. I.e., return true if unifying self with other would result in a feature structure equal to other.
Subsumes TotalRainfallMsg period year 1996 Month 06 subsumes TotalRainfallMsg period year 1996 month 06 attribute type 'RelativeVariation' magnitude unit 'inches' number 4 direction '+' TotalRainfallMsg period year 1996 month 06 attribute type 'RelativeVariation' magnitude unit 'inches' number 4 Does not subsume TotalRainfallMsg period year 1996 month 06
Using Subsumes ”Select from messages all DocPlans whose with a relType of Contrast and a nucleus which is a message of msgType ('TotalRainfallMsg')” d = DocPlan(relType = 'Contrast', nucleus = Message('TotalRainfallMsg')) return = filter(lambda msg: d.subsumes(msg), messages)
Implementation: Input Formats Messages: TotalRainfallMsg period year 1996 month 06 attribute type 'RelativeVariation' magnitude unit 'inches' number 4 direction '+'
Example Usage with open('msg_file', 'r') as f: msg_string = f.read() with open('rule_file', 'r') as f: rule_string = f.read() messages = read_messages(msg_string) rules = read_rules(rule_string) plan = bottom_up_plan(messages, rules)
Data Set - WeatherExplainer Simple example provided in [Reiter and Dale 2000] Created 3 messages and 3 rules in the input format
WeatherExplainer Messages TotalRainfallMsg period year 1996 month 06 attribute type 'RelativeVariation' magnitude unit 'inches' number 4 direction '+' MonthlyRainfallMsg period year 1996 month 06 attribute type 'RelativeVariation' magnitude unit 'inches' number 2 direction '+' MonthlyTemperatureMsg period year 1996 month 06 temperature category 'hot'
An Abstractive Summarization Pipeline Determine most relevant information and generate summary Extract all Information from input corpus Input Reviews Data Summary
ASSESS Testing Input: Review sentences tagged with crude-feature evaluations Crude-Feature to User-Defined-Feature mapping Simple content selection Group evaluations by UDF Calculate average evaluation Also include info on UDF-parent in hierarchy, number of evaluations
ASSESS Result It works! Evaluation of resulting DocPlan would say more about Rules and Content Selection than Document Structuring Algorithm Was able to handle larger number of messages and rules 4 of 5 rules used Still, only one message type used
Future Improvements Investigate whether this simple framework can be used to develop more “intelligent” rules for more sophisticated domain models [Carenini 2008] – SEA May require changes to implementation Complete comprehensive documentation and user- manual Submit to NLTK
References Bird, Steven; Ewan Klein; Edward Loper (2009). Natural Language Processing with Python. O'Reilly Media Inc. Print and online. Carenini, G., Moore, J.D., (2006) Generating and evaluating evaluative arguments. Artificial Intelligence, 170(11): 925- 952 Carenini, G., Ng, R., and Pauls, A. (2006) Multi-Document Summarization of Evaluative Text. Proc. of the Conf. of the European Chapter of the Association for Computational Linguistics. FitzGerald, N. (2009) A Complete Pipeline for Semantic Evaluation Summarization. Unpublished Project Report Lester, J. And Porter, B., (1997). Developing and empirically testing robust explanation generators: the KNIGHT experiments. Computational Linguistics, 23(1):65-101 Mann, W. and Thompson, S. (1988) Rhetorical structure theory: toward a functional theory of text organization. Text 3: 243-281. Marcu, D. (1997) From local to global coherence: A bottom-up approach to test planning. Proceedings of Fourteenth National Conference on Artificial Intelligence (AAAI-1997), 629-635. Pitler, Emily et al (2008). Easily Identifiable Discourse Relations. University of Pennsylvania Department of Computer and Information Science Technical Report No. MS-CIS-08-24. Reiter, E. and Dale, R. (1997) Building applied natural language generation systems. Natural Language Engineering 3 (1): 57-87. Reiter, E., and Robert Dale. Building Natural Language Generation Systems (Studies in Natural Language Processing). New York: Cambridge UP, 2000. Print. Young, R.M., Moore, J.D. DPOCL: A principled approach to discourse planning, in: Proceedings of the 7th International Workshop on Natural Language Generation, Kennebunkport, ME, June 17–21, 1994, pp. 13–20.