1 CPE 641 Natural Language Processing Asst. Prof. Nuttanart Facundes Text Classification Adapted from Barbara Rosario’s slides – Sept. 27,2004

2 Classification Text categorization (and other applications) Various issues regarding classification Clustering vs. classification, binary vs. multi-way, flat vs. hierarchical classification… Introduce the steps necessary for a classification task Define classes Label text Features Training and evaluation of a classifier

3 From: Foundations of Statistical Natural Language Processing. Manning and Schutze Classification Goal: Assign ‘objects’ from a universe to two or more classes or categories Examples: Problem Object Categories Tagging Word POS Sense Disambiguation Word The word’s senses Information retrieval Document Relevant/not relevant Sentiment classification Document Positive/negative Author identification Document Authors

4 Author identification They agreed that Mrs. X should only hear of the departure of the family, without being alarmed on the score of the gentleman's conduct; but even this partial communication gave her a great deal of concern, and she bewailed it as exceedingly unlucky that the ladies should happen to go away, just as they were all getting so intimate together. Gas looming through the fog in divers places in the streets, much as the sun may, from the spongey fields, be seen to loom by husbandman and ploughboy. Most of the shops lighted two hours before their time--as the gas seems to know, for it has a haggard and unwilling look. The raw afternoon is rawest, and the dense fog is densest, and the muddy streets are muddiest near that leaden-headed old obstruction, appropriate ornament for the threshold of a leaden-headed old corporation, Temple Bar.

5 Author identification Jane Austen ( ), Pride and Prejudice Charles Dickens ( ), Bleak House

6 Mosteller, Frederick and Wallace, David L Inference and Disputed Authorship: The Federalist. Author identification Federalist papers 77 short essays written in by Hamilton, Jay and Madison to persuade NY to ratify the US Constitution; published under a pseudonym The authorships of 12 papers was in dispute (disputed papers) In 1964 Mosteller and Wallace * solved the problem They identified 70 function words as good candidates for authorships analysis Using statistical inference they concluded the author was Madison

7 Function words for Author Identification

8

9 From: Foundations os Statistical Natural Language Processing. Manning and Schutze Classification Goal: Assign ‘objects’ from a universe to two or more classes or categories Examples: Problem Object Categories Author identification Document Authors Language identification Document Language

10 Language identification Tutti gli esseri umani nascono liberi ed eguali in dignità e diritti. Essi sono dotati di ragione e di coscienza e devono agire gli uni verso gli altri in spirito di fratellanza. Alle Menschen sind frei und gleich an Würde und Rechten geboren. Sie sind mit Vernunft und Gewissen begabt und sollen einander im Geist der Brüderlichkeit begegnen. Universal Declaration of Human RightsUniversal Declaration of Human Rights, UN, in 363 languages

11 Language identification égaux eguali iguales edistämään Ü ¿

12 From: Foundations of Statistical Natural Language Processing. Manning and Schutze Classification Goal: Assign ‘objects’ from a universe to two or more classes or categories Examples: Problem Object Categories Author identification Document Authors Language identification Document Language Text categorization Document Topics

13 Text categorization Topic categorization: classify the document into semantics topics The U.S. swept into the Davis Cup final on Saturday when twins Bob and Mike Bryan defeated Belarus's Max Mirnyi and Vladimir Voltchkov to give the Americans an unsurmountable 3-0 lead in the best-of-five semi-final tie. One of the strangest, most relentless hurricane seasons on record reached new bizarre heights yesterday as the plodding approach of Hurricane Jeanne prompted evacuation orders for hundreds of thousands of Floridians and high wind warnings that stretched 350 miles from the swamp towns south of Miami to the historic city of St. Augustine.

14 Text categorization Reuters Collection of (21,578) newswire documents. For research purposes: a standard text collection to compare systems and algorithms 135 valid topics categories

15 Reuters Top topics in Reuters

16 Reuters 2-MAR :51:43.42 livestock hog AMERICAN PORK CONGRESS KICKS OFF TOMORROW CHICAGO, March 2 - The American Pork Congress kicks off tomorrow, March 3, in Indianapolis with 160 of the nations pork producers from 44 member states determining industry positions on a number of issues, according to the National Pork Producers Council, NPPC. Delegates to the three day Congress will be considering 26 resolutions concerning various issues, including the future direction of farm policy and the tax law as it applies to the agriculture sector. The delegates will also debate whether to endorse concepts of a national PRV (pseudorabies virus) control and eradication program, the NPPC said. A large trade show, in conjunction with the congress, will feature the latest in technology in all areas of the industry, the NPPC added. Reuter

17 Text categorization: examples Topic categorization Reuters. Spam filtering Determine if a mail message is spam (or not) Customer service message classification

18 Classification vs. Clustering Classification assumes labeled data: we know how many classes there are and we have examples for each class (labeled data). Classification is supervised In Clustering we don’t have labeled data; we just assume that there is a natural division in the data and we may not know how many divisions (clusters) there are Clustering is unsupervised

19 Classification Class1 Class2

20 Classification Class1 Class2

21 Classification Class1 Class2

22 Classification Class1 Class2

23 Clustering

24 Clustering

25 Clustering

26 Clustering

27 Clustering

28 Categories (Labels, Classes) Labeling data 2 problems: Decide the possible classes (which ones, how many) Domain and application dependent Label text Difficult, time consuming, inconsistency between annotators

29 Reuters 2-MAR :51:43.42 livestock hog AMERICAN PORK CONGRESS KICKS OFF TOMORROW CHICAGO, March 2 - The American Pork Congress kicks off tomorrow, March 3, in Indianapolis with 160 of the nations pork producers from 44 member states determining industry positions on a number of issues, according to the National Pork Producers Council, NPPC. Delegates to the three day Congress will be considering 26 resolutions concerning various issues, including the future direction of farm policy and the tax law as it applies to the agriculture sector. The delegates will also debate whether to endorse concepts of a national PRV (pseudorabies virus) control and eradication program, the NPPC said. A large trade show, in conjunction with the congress, will feature the latest in technology in all areas of the industry, the NPPC added. Reuter Why not topic = policy ?

30 Binary vs. multi-way classification Binary classification: two classes Multi-way classification: more than two classes Sometime it can be convenient to treat a multi-way problem like a binary one: one class versus all the others, for all classes

31 Flat vs. Hierarchical classification Flat classification: relations between the classes undetermined Hierarchical classification: hierarchy where each node is the sub-class of its parent’s node

32 Single- vs. multi-category classification In single-category text classification each text belongs to exactly one category In multi-category text classification, each text can have zero or more categories

33 LabeledText LabeledText class in NLTK LabeledText LabeledText class >>> text = "Seven-time Formula One champion Michael Schumacher took on the Shanghai circuit Saturday in qualifying for the first Chinese Grand Prix." >>> label = “sport” >>> labeled_text = LabeledText(text, label) >>> labeled_text.text() “Seven-time Formula One champion Michael Schumacher took on the Shanghai circuit Saturday in qualifying for the first Chinese Grand Prix.” >>> labeled_text.label() “sport”

34 NLTK: The Classifier Interface classify classify determines which label is most appropriate for a given text token, and returns a labeled text token with that label. labels labels returns the list of category labels that are used by the classifier. >>> token = Token(“The World Health Organization is recommending more importance be attached to the prevention of heart disease and other cardiovascular ailments rather than focusing on treatment.”) >>> my_classifier.classify(token) “The World Health Organization is recommending more importance be attached to the prevention of heart disease and other cardiovascular ailments rather than focusing on treatment.”/ health >>> my_classifier.labels() ("sport", "health", "world",…)

35 Features >>> text = "Seven-time Formula One champion Michael Schumacher took on the Shanghai circuit Saturday in qualifying for the first Chinese Grand Prix." >>> label = “sport” >>> labeled_text = LabeledText(text, label) Here the classification takes as input the whole string What’s the problem with that? What are the features that could be useful for this example?

36 Feature terminology Feature: An aspect of the text that is relevant to the task Some typical features Words present in text Frequency of words Capitalization Are there NE? WordNet Others?

37 Feature terminology Feature: An aspect of the text that is relevant to the task Feature value: the realization of the feature in the text Words present in text : Kerry, Schumacher, China… Frequency of word: Kerry(10), Schumacher(1)… Are there dates? Yes/no Are there PERSONS? Yes/no Are there ORGANIZATIONS? Yes/no WordNet: Holonyms (China is part of Asia), Synonyms(China, People's Republic of China, mainland China)

38 Feature Types Boolean (or Binary) Features Features that generate boolean (binary) values. Boolean features are the simplest and the most common type of feature. f 1 (text) = 1 if text contain “Kerry” 0 otherwise f 2 (text) = 1 if text contain PERSON 0 otherwise

39 Feature Types Integer Features Features that generate integer values. Integer features can be used to give classifiers access to more precise information about the text. f 1 (text) = Number of times text contains “Kerry” f 2 (text) = Number of times text contains PERSON

40 Features in NLTK Feature Detectors Features can be defined using feature detector functions, which map LabeledTexts to values Method: detect, which takes a labeled text, and returns a feature value.detect >>> def ball(ltext): return (“ball” in ltext.text()) >>> fdetector = FunctionFeatureDetector(ball) >>> document1 = "John threw the ball over the fence".split() >>> fdetector.detect(LabeledText(document1) 1 >>> document2 = "Mary solved the equation".split() >>> fdetector.detect(LabeledText(document2) 0

41 Features in NLTK Feature Detector Lists: data structures that represent the feature detector functions for a set of features. Feature Value Lists

42 Feature selection How do we choose the “right” features?

43 Classification Define classes Label text Extract Features Choose a classifier >>> my_classifier.classify(token) The Naive Bayes Classifier NN (perceptron) SVM …. Train it (and test it) Use it to classify new examples

44 Training (We’ll see what we mean exactly with training when we’ll talk about the algorithms) Adaptation of the classifier to the data Usually the classifier is defined by a set of parameters Training is the procedure for finding a “good” set of parameters Goodness is determined by an optimization criterion such as misclassification rate Some classifiers are guaranteed to find the optimal set of parameters

45 Testing, evaluation of the classifier After choosing the parameters of the classifiers (i.e. after training it) we need to test how well it’s doing on a test set (not included in the training set) Calculate misclassification on the test set

46 Evaluating classifiers Contingency table for the evaluation of a binary classifier GREEN is correctRED is correct GREEN was assignedab RED was assignedcd Accuracy = (a+d)/(a+b+c+d) Precision: P_GREEN = a/(a+b), P_ RED = d/(c+d) Recall: R_GREEN = a/(a+c), R_ RED = d/(b+d)

47 *From: Improving the Performance of Naive Bayes for Text Classification, Shen and Yang Training size The more the better! (usually) Results for text classification *

48 *From: Improving the Performance of Naive Bayes for Text Classification, Shen and Yang Training size

49 *From: Improving the Performance of Naive Bayes for Text Classification, Shen and Yang Training size

50 Authorship Attribution a Comparison Of Three Methods, Matthew Care Training Size Author identification