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pythonnlpnltkdocument-classification

NLTK - Multi-labeled Classification

I am using NLTK, to classify documents - having 1 label each, with there being 10 type of documents.

For text extraction, I am cleaning text (punctuation removal, html tag removal, lowercasing), removing nltk.corpus.stopwords, as well as my own collection of stopwords.

For my document feature I am looking across all 50k documents, and gathering the top 2k words, by frequency (frequency_words) then for each document identifying which words in the document that are also in the global frequency_words.

I am then passing in each document as hashmap of {word: boolean} into the nltk.NaiveBayesClassifier(...) I have a 20:80 test-training ratio in regards to the total number of documents.

The issues I am having:

  1. Is this classifier by NLTK, suitable to multi labelled data? - all examples I have seen are more about 2-class classification, such as whether something is declared as a positive or negative.
  2. The documents are such that they should have a set of key skills in - unfortunately I haven't got a corpus where these skills lie. So I have taken an approach with the understanding, a word count per document would not be a good document extractor - is this correct? Each document has been written by individuals, so I need to leave way for individual variation in the document. I am aware SkLearn MBNaiveBayes which deals with word count.
  3. Is there an alternative library I should be using, or variation of this algorithm?

Thanks!

Solution

  • Terminology: Documents are to be classified into 10 different classes which makes it a multi-class classification problem. Along with that if you want to classify documents with multiple labels then you can call it as multi-class multi-label classification.

    For the issues which you are facing,

    1. nltk.NaiveBayesClassifier() is a out-of-box multi-class classifier. So yes you can use this to solve this problem. As per the multi-labelled data, if your labels are a,b,c,d,e,f,g,h,i,j then you have to define label 'b' of a particular document as '0,1,0,0,0,0,0,0,0,0'.

    2. Feature extraction is the hardest part of Classification (Machine learning). I recommend you to look into different algorithms to understand and select the one best suits for your data(without looking at your data, it is tough to recommend which algorithm/implementation to use)

    3. There are many different libraries out there for classification. I personally used scikit-learn and i can say it was good out-of-box classifier.

    Note: Using scikit-learn, i was able to achieve results within a week, given data set was huge and other setbacks.