Natural Language Provessing - Handling Narrarive Fields in Datasets for Classification

1. Handling Narrative Fields in Datasets for Classification Portland Data Science Group Created by Andrew Ferlitsch Community Outreach Officer May, 2017

2. Typical Dataset Feature 1 Feature 2 Feature 3 Feature 4 Label real-value real-value real-value categorical-value category real-value real-value real-value categorical-value category real-value real-value real-value categorical-value category Dataset Clean Categorical Value Conversion Feature Scaling Progression in Dataset Preparation

3. Feature Reduction • Filter out Garbage (dirty data) • Filter out Noise (non-relevant features) • Goal = Low Bias, Low Variance Data + Noise + Garbage Relevant Data Only Information Gain Reduce Entropy

4. Dataset with Narrative Fields Feature 1 Feature 2 Feature 3 Feature 4 Label real-value real-value narrative categorical-value category real-value real-value narrative categorical-value category real-value real-value narrative categorical-value category Narrative is plain text which is a human description of the entry, i.e., what happened. “upon arrival, the individual was initially non-responsive. …” Category (label) is a classification based on the narrative by a human interpretation. 012 // Code value for “coarse” category

5. Problem with Narrative Text Fields • Examples: 911 calls, Police/Emergency/Medical, Incidents, Inspections, Surveys, Complaints, Reviews – Human Entered – Human Interpreted => Categorizing – Different People Entering and Categorizing – Non-Uniformity – Human Errors

6. Challenge • Convert Narrative Fields into Features with Categorical ( or preferably Real) Values. Data + Narrative Data + Categorical / Real Values

7. Bag of Words Bag of Words Narrative Field • Unordered List of Words • Convert Unique Words in Categorical Variables • Set 1 if word appears in narrative; otherwise set 0.

8. Cleansing and Tokenize (Words) • Remove Punctuation • Expand Contractions (e.g., isn’t -> is not) • Lowercase The quick brown fox jumped over the lazy dog. the:2 quick:1 brown:1 fox:1 Jumped:1 over:1 lazy:1 dog:1

9. Narrative as Categorical Variables The quick brown fox jumped over the lazy dog. The dog barked while the cat was jumping. the quick brow n fox jump ed over lazy dog bark ed while cat was jum ping 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 Issues: Explosion of categorical variables. For example, if the dataset has 80,000 unique words, then you would have 80,000 categorical variables!

10. Corpus • A collection of related documents. • The Narratives in the Dataset are the Corpus. • Each Narrative is a Document Feature 1 .. N Narrative Label CORPUS Document

11. Word Distribution • Make a pass through all the narratives (corpus) building a dictionary. • Sort by Word Frequency (number of times it occurs). 0 MAX Upper Threshold Lower Threshold Useless Words – Have no significance (e.g. the) Commonly used Words Rare Words or Misspellings

12. Stop Word Removal • Remove Highest Frequency Words (above upper threshold), and • Remove Lowest Frequency Words (below lower threshold) (optional). The quick brown fox jumped over the lazy dog. The dog barked while the cat was jumping. quick brown fox jumped lazy dog barked cat jumpin g 1 1 1 1 1 1 1 1 1 1 Well known predefined Stop Word Lists – most widely used is the Porter List

13. Stemming • Stemming – Reduce words to their root stem. Ex. Jumped, jumping, jumps => jump • Does not use predefined dictionary. Uses grammar ending rules. jumped, jumping barked quick brown fox jump lazy dog bark cat 1 1 1 1 1 1 1 1 1 1

14. Lemmatization • Stems are correct if word is not exception, BUT incorrect when word is an exception. Ex. something => someth • Lemmatization means reducing words to their root form, but correcting the exceptions by using a dictionary of common exceptions (vs. all words, e.g., 1000 words instead of 100,000).

15. Term Frequency (TF) • Issue: All words are weighted the same • Term Frequency is weighting the frequency of the word in the corpus, and using the frequency as its feature value (vs. 1 or 0). (no. of occurrences in corpus) / (no. of unique words in corpus) quick brown fox jump lazy dog bark cat 0.001 0.003 0.0002 0.006 0.0001 0.007 0.0001 0.007 0.006 0.007 0.0001 0.007

16. Inverse Document Frequency (IDF) • Issue: TF gives higher weight to words that are the most frequently used – may result in underfitting (too general). • Inverse Document Frequency is weighted words by have rarely they appear in the corpus (assumption is the word is more significant in a document). log ((no. of unique words in corpus) / (no. of occurrences in corpus) ) quick brown fox jump lazy dog bark cat 2 1.5 2.7 1.2 3 1.15 3 1.15 1.2 1.15 3 1.15

17. Pruning • Even with Stemming/Lemmatization, the feature matrix will be massive in size (e.g, 30,000 features). • Reduce to smaller number – typically 500 to 1000. • Choose the highest TF or IDF values in the Corpus.

18. Advance Topic – Word Reduction • Words that are part of a common grouping are replaced with a root word for the group. • Steps: 1. Stemming/Lemmatization 2. Lookup Root Word in Word Group Dictionary 3. If entry exists, replace with common root word for the group. Group Example: male: [ man, gentleman, boy, guy, dude ]

19. Advance Topic – Word Reduction male : [ man, gentleman, boy, guy, dude ] female: [ woman, lady, girl, gal ] parent : [ father, mother, mom, mommy, dad, daddy ] Word Root man male gentleman male boy male guy male dude male woman female Lady Female girl female gal female The mother played with the girls while the dad prepared snacks for the ladies in mom’s reading group. parent, play, female, parent, prepare, snack, female, parent, read, group

20. Advance Topics – N-grams • Instead of parsing the sentence into single words, each as a feature, we group them in pairs (2-gram) or triplets (3-grams), etc, …. • Parameters: 1. Choose Window Size (2, 3, …) 2. Choose Stride Length (1, 2, …) 2-gram word1 word2 word3 … word4 stride of 1 2-gram

21. Advance Topics – N-grams The quick brown fox jumped over the lazy dog quick, brown, fox, jump, lazy, dog 2-grams, stride of 1 quick, brown brown, fox fox, jump jump, lazy lazy, dog Dog, <null> quick, brown brown, fox fox, jump Jump, lazy Lazy, dog dog 1 1 1 1 1 1

22. More – Not Covered • Word-Vectors [Word Embedding] • Correcting Misspellings • Detecting incorrectly categorized Narratives.

23. Final – Homegrown Tool • I built a command tool for doing all the steps in this presentation. • Java based, packaged as a JAR file. https://github.com/andrewferlitsch/Portland-Data-Science-Group/blob/master/README.NLP.md

24. Final – Homegrown Tool - Examples • Quora question pairs (training set: 400,000) java –jar nlp.jar –c3,4 train.csv • Remove Step Words java –jar nlp.jar –c3,4 -e p train.csv • Lemma and Reduce to Common Root java –jar nlp.jar –c3,4 -e p –l –r train.csv • Lemma and Reduce to Common Root java –jar nlp.jar –c3,4 -e p –l –r –F train.csv

Natural Language Provessing - Handling Narrarive Fields in Datasets for Classification

Natural Language Provessing - Handling Narrarive Fields in Datasets for Classification

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