06-Summarization

Summarization

TL;DR

Text summarization
Most important technique
- Extraction
Tasks:
- Key word extraction
- Sentence extraction
Algorithms:
- Supervised
- Unsupervised
Abstract summarization still an open problem

Introduction

What is Summarization?

Reduce natural language text document
Goal: Compress text by extracting the most important/relevant parts 💪

Applications

Articles, news: Outlines or abstracts
Email / Email threads
Health information
Meeting summarization

Dimensions

Single vs. multiple
- Single-document summarization
  - Given single document
  - Produce abstract, outline, headline, etc.
- Multiple-document summarization
  - Given a group of documents
  - A series of news stories on the same event
  - A set of web pages about some topic or question
Generic vs. Query-focused summarization
- Generic summarization: summarize the content of the document
- Query-focused summarization: kind of complex question answering 🤪
  - Summarize a document with respect to an information need expressed in a user query
  - Longer, descriptive, more informative answers
  - Answer a question by summarizing a document that has information to construct the answer

Techniques

Extraction

Select subset of existing text segments
e.g.:
- Sentence extraction
- Key-phrase extraction
Simpler, most focus in research

Abstraction

Use natural language generation to create summary
More human like

Extractive summarization

Three main components

Content selection ("Which parts are important to be in the summary?")
Information ordering ("How to order summaries?")
Sentence realization (Clean up/Simplify sentences)

Supervised approaches

Key-word extraction

Given: Text (e.g. abstract of an article, …)
Task: Find most important key phrases
Computer Human
Select key phrases from the text Abstraction of the text
No new wordings New words

Computer	Human
Select key phrases from the text	Abstraction of the text
No new wordings	New words

Key-phrase extraction using Supervised approaches

Given: Collection of text with key-words
Algorithm
- Extract all uni-grams, bi-grams and tri-grams
  - Example
    Extraction: Compatibility, Compatibility of, Compatibility of systems, of, of systems, of systems of, systems, systems of, systems of linear, of linear, of linear constraints, linear, linear constraints, linear constraints over, …
- Annotate each examples with features
- Annotate training examples with class:
  - 1 if sequence is part of the key words
  - 0 if sequence is not part of the key words
- Train classifier
- Create test examples and classify
Examples set
- All uni-, bi-, and trigrams (except punctuation)
- restrict to certain POS sets
🔴 Problem:
- Enough examples to generate all/most key phrases
- Too many examples -> low performance of classifier
Features
- Term frequency
- TF-IDF (Term Frequency–Inverse Document Frequency)
  - Reflect importance of a word in a document
    $$ \text{TF-IDF} = tf * idf $$
    - $tf(w, D)$:
      - Term Frequency, measures how frequently a term occurs in a document.
      - Number of occurrences of word $w$ in document $d$ divided by the maximum frequency of one word in $D$
      $$ t f(w, D)=\\#(w, D) \frac{\\#(w, D)}{\max\_{w^{\prime} \in D}\left(w^{\prime}, D\right)} $$
      Alternative definition:
      $$ > tf(w, D) = \frac{\text{count of } w \text{ in } D}{\text{number of words in } D} > $$
    - $idf(w)$:
      - Inverse Document Frequency, measures how important a term is
      - Idea: Words which occur in less documents are more important
      - Number of documents divided by the number of documents which contain $w$
      $$ i d f(w)=\log \frac{|D|}{|\\{d \in D: w \in d\\}|} $$
  - Example:
    - Consider a document containing 100 words wherein the word cat appears 3 times. The term frequency (i.e., tf) for cat is then (3 / 100) = 0.03. Now, assume we have 10 million documents and the word cat appears in one thousand of these. Then, the inverse document frequency (i.e., idf) is calculated as log(10,000,000 / 1,000) = 4. Thus, the Tf-idf weight is the product of these quantities: 0.03 * 4 = 0.12.
- Length of the example
- Relative position of the first occurrence
- Boolean syntactic features
  - contains all caps
Learning algorithm
- Decision trees,
- Naive Bayes classifier
- …
Evaluation
- Compare results to reference
  - Test set: Text
  - Human generated Key words
- Metrics:
  - Precision
  - Recall
  - F-Score
- 🔴 Problems
  - Humans do not only extract key words, but also abstract
  - Normally not all key words are reachable

Sentence extraction

Use statistic heuristics to select sentences
Do not change content and meaning
💡 Idea
- Use measure to determine importance of sentence
  - TF-IDF
  - Supervised trained combination of several features
- Rank sentence according to metric
- Output sentences with highest scores:
  - Fixed number
  - All sentence above threshold
Limitations: Do NOT change text (e.g. add phrases, delete parts of the text)
Evaluation
- Idea: Compare automatic summary to abstract of text
  - Problem: Different sentences –> Nearly no exact match 😭
- ROUGE - Recall-Oriented Understudy for Gisting Evaluation
  - Use also approximate matches
  - Compare automatic summary to human generated text
    - Given a document D and an automatic summary X
      - M humans produce a set of reference summaries of D
      - What percentage of the n-grams from the reference summaries appear in X?
  - ROUGE-N: Overlap of N-grams between the system and reference summaries
    $$ \text{ROUGH-N} = \frac{\sum\_{S \in \\{\text{Reference Summaries}\\}} \sum\_{gram\_n \in S}\operatorname{Count}\_{match}(gram\_n)}{\sum\_{S \in \\{\text{Reference Summaries}\\}} \sum\_{gram\_n \in S}\operatorname{Count}(gram\_n)} $$
    - Example:
      Auto-generated summary ($Y$)
      the cat was found under the bed
      Gold standard (human produced) ($X1$)
      the cat was under the bed
      1-gram and 2-gram summary:
      # 1-gram reference 1-gram 2-gram reference 2-gram
      1 the the the cat the cat
      2 cat cat cat was cat was
      3 was was was found was under
      4 found under found under under the
      5 under the under the the bed
      6 the bed the bed
      7 bed
      count 7 6 6 5
      - $\operatorname{ROUGE}-1(X1, Y) = \frac{6}{6} = 1$
      - $\operatorname{ROUGE}-2(X1, Y) = \frac{4}{5}$

#	1-gram	reference 1-gram	2-gram	reference 2-gram
1	the	the	the cat	the cat
2	cat	cat	cat was	cat was
3	was	was	was found	was under
4	found	under	found under	under the
5	under	the	under the	the bed
6	the	bed	the bed
7	bed
count	7	6	6	5

Unsupervised approaches

Problems of supervised approaches: Hard to acquire training data

We try to use unsupervised learning to find key phrases / sentences which are most important. But which sentences are most important?

💡 Idea: Sentences which are most similar to the other sentences in the text

Graph-based approaches

Map text into a graph
- Nodes:
  - Text segments: Words
- Edges: Similarity
Find most important/central vertices
Algorithms: TextRank / LexRank

Graph-based approaches : Key-phrase extraction

Graph
- Nodes
  - Text segments: Words
  - Restriction:
    - Nouns
    - Adjective
- Edges
  - items co-occur in a window of N words
Calculate most important nodes
Build Multi-word expression in post-processing
- Mark selected items in original text
- If two adjacent words are marked –> Collapse to one multi-words expression
Example

Graph-based approaches : Sentence extraction

Graph:

Nodes: Sentences
Edges: Fully connected with weights
Weights:
- TextRank: Word overlap normalized to sentence length
  $$ \text {Similarity}\left(S\_{i}, S\_{j}\right)=\frac{\left|\left\\{w\_{k} \mid w\_{k} \in S\_{i} \text{ & } w\_{k} \in S\_{j}\right\\}\right|}{\log \left(\left|S\_{i}\right|\right)+\log \left(\left|S\_{j}\right|\right)} $$
- LexRank: Cosine Similarity of TF-IDF vectors
  $$ \text { idf-modified-cosine }(x, y)=\frac{\sum\_{w \in x, y} \mathrm{tf}\_{w, x} \mathrm{tf}\_{w, y}\left(\mathrm{idf}\_{w}\right)^{2}}{\sqrt{\sum\_{x\_{i} \in x}\left(\mathrm{tf}\_{x\_{i}, x} \mathrm{idf}\_{x\_{i}}\right)^{2}} \times \sqrt{\sum\_{y\_{i} \in y}\left(\mathrm{tf}\_{y\_{i}, y} \mathrm{idf}\_{y\_{i}}\right)^{2}}} $$