Starting Part-of-Speech Tagging

This is by no means the latest on the subject of probabilistic part-of-speech tagging of documents but nevertheless provides a good starting point to look at the basic model along with training and testing data.

This paper [1] takes a stab at comparing unsupervised HMM-based POS taggers. It is quite strange that something as simple as an HMM can be expected to tag words according to parts of speech without any supervision. By general standards of classification accuracy, we find that an unsupervised HMM does in fact perform poorly: [1] shows the average greedy 1-to-1 accuracy (explained in last section) of 51\% at best and the paper we looked at last time also gives the same performance on this data.

Clearly, some supervision is required. Though it isn’t the main purpose of the paper from last time, two approaches are taken. The first is to consider closed-class words. These are words which amount to a handful having a common tag such as punctuation, pronouns, and possessive markers while open classes are things like verbs and nouns. The authors modify the models by fixing the class of these closed-class words by giving them zero probability to tags they definitely don’t belong to. The results improve to about 58\% which is not a huge jump.

The second thing they try is to use the hidden representation from an unsupervised run of a HMM (in addition to other simple features) and feed them as features to a classification model. I am not too interested in this but suffice it to say accuracy jumps up yet again to about 85\%.


Here are some links to high-quality data used in these papers.

Evaluation Details

Luckily, anything falling into the category of classification is by far the easiest to evaluate in a completely intuitive and concrete way. It’s not as easy to fool someone with shady looking experiments as it is with unsupervised models without a classification objective.

The method employed in this paper is to split the data into a training and test set. Then, from the model inferred from the training set a greedy 1-to-1 map is established between each hidden state with a part-of-speech tag (another approach would be find a best-matching instead). This mapping is employed to label the test data and compare against the treebanks.

Another method is to allow a 1-to-Many mapping where more than one hidden state could be mapped to a tag by simply sending each hidden state to the best matching tag. The results with this approach seem to show higher accuracies.

[1] Jianfeng Gao and Mark Johnson. 2008. “A comparison of Bayesian estimators for unsupervised Hidden Markov Model POS taggers”. Empirical Methods in Natural Language Processing

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