Classification with Naive Bayes
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A Deep Dive into Classification with Naive Bayes. Along the way we take a look at some basics from Ian Witten's Data Mining book and dig into the algorithm....

A Deep Dive into Classification with Naive Bayes. Along the way we take a look at some basics from Ian Witten's Data Mining book and dig into the algorithm.

Presented on Wed Apr 27 2011 at SeaHUG in Seattle, WA.

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  • https://cwiki.apache.org/MAHOUT/books-tutorials-and-talks.html
  • Contrasts with “1Rule” method (1Rule uses 1 attribute)NB allows all attributes to make contributions that are equally important and independent of one another
  • This classifier produces a probability estimate for each class rather than a predictionConsidered “Supervised Learning”
  • comparison with other classification methods in 2006 showed that Bayes classification is outperformed by more current approaches, such as boosted trees or random forestsAn advantage of the naive Bayes classifier is that it requires a small amount of training data to estimate the parameters (means and variances of the variables) necessary for classification.
  • Pr[E|H] -> all evidence for instances with H->”yes”Pr[H] -> percent of instances w/ this outcomePr[E] -> sum of the values ( ) for all outcomes
  • Book reference: snow crashFor each attribute “a” there are multiple values, and given these combinations we need to look at how many times the instances were actually classified each class.In training we use the term “outcome”, in classification we use the term “class”Example: say we have 2 attributes to an instance
  • We don’t take into account some of the other things like “missing values” here
  • Now that we’ve established the case for Naïve Bayes + Text  show how it fits in with other classifications algos
  • *** Need to sell case for using another feature calculating mechanic ***when one class has more training examples than anotherNaive Bayes selects poor weights for the decision boundary. To balance the amount of training examples used per estimatethey introduced a “complement class” formulation of Naive Bayes.A document is treated as a sequence of words and it is assumed that each word position is generated independently of every other word
  • Term frequency =num occurrences of the considered term ti in document dj / sizeof ( words in doc dj )Normalized to protect against bias in larger docsIDF = log( Normalized Frequency for a term(feature) in a document is calculated by dividing the term frequency by the root mean square of terms frequencies in that documentWeight Normalized Tffor a given feature in a given label = sum of Normalized Frequency of the feature across all the documents in the label.
  • Need to get a better handle on Sigma_kirSigmaWijhttps://cwiki.apache.org/MAHOUT/bayesian.html
  • https://cwiki.apache.org/confluence/display/MAHOUT/Twenty+Newsgroups
  • Can also test sequentially

Classification with Naive Bayes Presentation Transcript

  • 1. Classification with Naïve Bayes
    A Deep Dive into Apache Mahout
  • 2. Today’s speaker – Josh Patterson
    josh@cloudera.com / twitter: @jpatanooga
    Master’s Thesis: self-organizing mesh networks
    Published in IAAI-09: TinyTermite: A Secure Routing Algorithm
    Conceived, built, and led Hadoop integration for the openPDC project at TVA (Smartgrid stuff)
    Led small team which designed classification techniques for time series and Map Reduce
    Open source work at http://openpdc.codeplex.com
    Now: Solutions Architect at Cloudera
    2
  • 3. What is Classification?
    Supervised Learning
    We give the system a set of instances to learn from
    System builds knowledge of some structure
    Learns “concepts”
    System can then classify new instances
  • 4. Supervised vs Unsupervised Learning
    Supervised
    Give system examples/instances of multiple concepts
    System learns “concepts”
    More “hands on”
    Example: Naïve Bayes, Neural Nets
    Unsupervised
    Uses unlabled data
    Builds joint density model
    Example: k-means clustering
  • 5. Naïve Bayes
    Called Naïve Bayes because its based on “Baye’sRule” and “naively” assumes independence given the label
    It is only valid to multiply probabilities when the events are independent
    Simplistic assumption in real life
    Despite the name, Naïve works well on actual datasets
  • 6. Naïve Bayes Classifier
    Simple probabilistic classifier based on
    applying Baye’s theorem (from Bayesian statistics)
    strong (naive) independence assumptions.
    A more descriptive term for the underlying probability model would be “independent feature model".
  • 7. Naïve Bayes Classifier (2)
    Assumes that the presence (or absence) of a particular feature of a class is unrelated to the presence (or absence) of any other feature.
    Example:
    a fruit may be considered to be an apple if it is red, round, and about 4" in diameter.
    Even if these features depend on each other or upon the existence of the other features, a naive Bayes classifier considers all of these properties to independently contribute to the probability that this fruit is an apple.
  • 8. A Little Bit o’ Theory
  • 9. Condensing Meaning
    To train our system we need
    Total number input training instances (count)
    Counts tuples:
    {attributen,outcomeo,valuem}
    Total counts of each outcomeo
    {outcome-count}
    To Calculate each Pr[En|H]
    ({attributen,outcomeo,valuem} / {outcome-count} )
    …From the Vapor of That Last Big Equation
  • 10. A Real Example From Witten, et al
  • 11. Enter Apache Mahout
    What is it?
    Apache Mahout is a scalable machine learning library that supports large data sets
    What Are the Major Algorithm Type?
    Classification
    Recommendation
    Clustering
    http://mahout.apache.org/
  • 12. Mahout Algorithms
  • 13. Naïve Bayes and Text
    Naive Bayes does not model text well.
    “Tackling the Poor Assumptions of Naive Bayes Text Classifiers”
    http://people.csail.mit.edu/jrennie/papers/icml03-nb.pdf
    Mahout does some modifications based around TF-IDF scoring (Next Slide)
    Includes two other pre-processing steps, common for information retrieval but not for Naive Bayes classification
  • 14. High Level Algorithm
    For Each Feature(word) in each Doc:
    Calc: “Weight Normalized Tf-Idf”
    for a given feature in a label is the Tf-idf calculated using standard idf multiplied by the Weight Normalized Tf
    We calculate the sum of W-N-Tf-idf for all the features in a label called Sigma_k, and alpha_i == 1.0
    Weight = Log [ ( W-N-Tf-Idf + alpha_i ) / ( Sigma_k + N ) ]
  • 15. BayesDriver Training Workflow
    Naïve Bayes Training MapReduce Workflow in Mahout
  • 16. Logical Classification Process
    Gather, Clean, and Examine the Training Data
    Really get to know your data!
    Train the Classifier, allowing the system to “Learn” the “Concepts”
    But not “overfit” to this specific training data set
    Classify New Unseen Instances
    With Naïve Bayes we’ll calculate the probabilities of each class wrt this instance
  • 17. How Is Classification Done?
    Sequentially or via Map Reduce
    TestClassifier.java
    Creates ClassifierContext
    For Each File in Dir
    For Each Line
    Break line into map of tokens
    Feed array of words to Classifier engine for new classification/label
    Collect classifications as output
  • 18. A Quick Note About Training Data…
    Your classifier can only be as good as the training data lets it be…
    If you don’t do good data prep, everything will perform poorly
    Data collection and pre-processing takes the bulk of the time
  • 19. Enough Math, Run the Code
    Download and install Mahout
    http://www.apache.org
    Run 20Newsgroups Example
    https://cwiki.apache.org/confluence/display/MAHOUT/Twenty+Newsgroups
    Uses Naïve Bayes Classification
    Download and extract 20news-bydate.tar.gz from the 20newsgroups dataset
  • 20. Generate Test and Train Dataset
    Training Dataset:
    mahout org.apache.mahout.classifier.bayes.PrepareTwentyNewsgroups
    -p examples/bin/work/20news-bydate/20news-bydate-train
    -o examples/bin/work/20news-bydate/bayes-train-input
    -a org.apache.mahout.vectorizer.DefaultAnalyzer
    -c UTF-8
    Test Dataset:
    mahout org.apache.mahout.classifier.bayes.PrepareTwentyNewsgroups
    -p examples/bin/work/20news-bydate/20news-bydate-test
    -o examples/bin/work/20news-bydate/bayes-test-input
    -a org.apache.mahout.vectorizer.DefaultAnalyzer
    -c UTF-8
  • 21. Train and Test Classifier
    Train:
    $MAHOUT_HOME/bin/mahout trainclassifier
    -i 20news-input/bayes-train-input
    -o newsmodel
    -type bayes
    -ng 3
    -source hdfs
    Test:
    $MAHOUT_HOME/bin/mahout testclassifier
    -m newsmodel
    -d 20news-input
    -type bayes
    -ng 3
    -source hdfs
    -method mapreduce
  • 22. Other Use Cases
    Predictive Analytics
    You’ll hear this term a lot in the field, especially in the context of SAS
    General Supervised Learning Classification
    We can recognize a lot of things with practice
    And lots of tuning!
    Document Classification
    Sentiment Analysis
  • 23. Questions?
    We’re Hiring!
    Cloudera’sDistro of Apache Hadoop:
    http://www.cloudera.com
    Resources
    “Tackling the Poor Assumptions of Naive Bayes Text Classifiers”
    http://people.csail.mit.edu/jrennie/papers/icml03-nb.pdf