Text Categorization

1
Text Categorization

• Assigning documents to a fixed set of categories
• Applications
• Web pages
• Recommending pages
• Yahoo-like classification hierarchies
• Categorizing bookmarks
• Newsgroup Messages /News Feeds / Micro-blog Posts
• Recommending messages, posts, tweets, etc.
• Message filtering
• News articles
• Personalized news
• Email messages
• Routing
• Folderizing
• Spam filtering

2
Learning for Text Categorization

• Text Categorization is an application of
  classification
• Typical Learning Algorithms
• Bayesian (naïve)
• Neural network
• Relevance Feedback (Rocchio)
• Nearest Neighbor
• Support Vector Machines (SVM)

3
Nearest-Neighbor Learning Algorithm

• Learning is just storing the representations of
  the training examples in data set D
• Testing instance x
• Compute similarity between x and all examples in
  D
• Assign x the category of the most similar
  examples in D
• Does not explicitly compute a generalization or
  category prototypes (i.e., no modeling)
• Also called
• Case-based
• Memory-based
• Lazy learning

4
K Nearest-Neighbor

• Using only the closest example to determine
  categorization is subject to errors due to
• A single atypical example.
• Noise (i.e. error) in the category label of a
  single training example.
• More robust alternative is to find the k
  most-similar examples and return the majority
  category of these k examples.
• Value of k is typically odd to avoid ties, 3 and
  5 are most common.

5
Similarity Metrics

• Nearest neighbor method depends on a similarity
  (or distance) metric
• Simplest for continuous m-dimensional instance
  space is Euclidian distance
• Simplest for m-dimensional binary instance space
  is Hamming distance (number of feature values
  that differ)
• For text, cosine similarity of TF-IDF weighted
  vectors is typically most effective

6
Basic Automatic Text Processing

• Parse documents to recognize structure and
  meta-data
• e.g. title, date, other fields, html tags, etc.
• Scan for word tokens
• lexical analysis to recognize keywords, numbers,
  special characters, etc.
• Stopword removal
• common words such as the, and, or which are
  not semantically meaningful in a document
• Stem words
• morphological processing to group word variants
  (e.g., compute, computer, computing,
  computes, can be represented by a single stem
  comput in the index)
• Assign weight to words
• using frequency in documents and across documents
• Store Index
• Stored in a Term-Document Matrix (inverted
  index) which stores each document as a vector of
  keyword weights

7
tf x idf Weighs

• tf x idf measure
• term frequency (tf)
• inverse document frequency (idf) -- a way to deal
  with the problems of the Zipf distribution
• Recall the Zipf distribution
• Want to weight terms highly if they are
• frequent in relevant documents BUT
• infrequent in the collection as a whole
• Goal assign a tf x idf weight to each term in
  each document

8
tf x idf
9
Inverse Document Frequency

• IDF provides high values for rare words and low
  values for common words

10
tf x idf Example
  Doc 1 Doc 2 Doc 3 Doc 4 Doc 5 Doc 6   df idf log2(N/df)
T1 0 2 4 0 1 0   3 1.00
T2 1 3 0 0 0 2   3 1.00
T3 0 1 0 2 0 0   2 1.58
T4 3 0 1 5 4 0   4 0.58
T5 0 4 0 0 0 1   2 1.58
T6 2 7 2 1 3 0   5 0.26
T7 1 0 0 5 5 1   4 0.58
T8 0 1 1 0 0 3   3 1.00
The initial Term x Doc matrix(Inverted Index)
Documents represented as vectors of words
  Doc 1 Doc 2 Doc 3 Doc 4 Doc 5 Doc 6
T1 0.00 2.00 4.00 0.00 1.00 0.00
T2 1.00 3.00 0.00 0.00 0.00 2.00
T3 0.00 1.58 0.00 3.17 0.00 0.00
T4 1.74 0.00 0.58 2.90 2.32 0.00
T5 0.00 6.34 0.00 0.00 0.00 1.58
T6 0.53 1.84 0.53 0.26 0.79 0.00
T7 0.58 0.00 0.00 2.92 2.92 0.58
T8 0.00 1.00 1.00 0.00 0.00 3.00
tf x idfTerm x Doc matrix
11
K Nearest Neighbor for Text
Training For each each training example ltx,
c(x)gt ? D Compute the corresponding TF-IDF
vector, dx, for document x Test instance
y Compute TF-IDF vector d for document y For
each ltx, c(x)gt ? D Let sx cosSim(d,
dx) Sort examples, x, in D by decreasing value of
sx Let N be the first k examples in D. (get
most similar neighbors) Return the majority class
of examples in N