Chambers and Jurafsky, Jointly combining implicit constraints improves temporal ordering, EMNLP 2008

Citation

Jointly combining implicit constraints improves temporal ordering, by N. Chambers, D. Jurafsky. In Proceedings of the Conference on Empirical Methods in Natural Language Processing, 2008.

Online version

This paper is available online [1].

Summary

This paper presents the idea of using global constraints to better inform local decisions on temporal ordering of events in text. Two types of global constraints are used: transitivity (A before B and B before C implies A before C) and time expression normalization (e.g. last Tuesday is before today).

The constraints are first used to create more densely connected temporal network of events. Then they are enforced over this temporal network of events using Integer Linear Programming to ensure global consistency of local ordering.

This is an early and influential paper presenting an unsupervised approach to review classification. There are three basic ideas introduced here.

One key idea is to score the polarity of a review based on the total polarity of the phrases in it.

A second idea is to use patterns of part of speech tags to pick out phrases that are likely to be meaningful and unambiguous with respect to semantic orientation (e.g. ADJ NOUN might pick out "good service" or "delicious desserts").

Finally, these potentially-meaningful phrases are then scored using pointwise mutual information (PMI) to seed words on known polarity. Specifically, Turney uses PMI to compare each phrase to the words "excellent" or "poor", and then uses these distances to give an overall score for the polarity to each phrase, based on the difference of its PMI with "excellent" to the PMI with "poor". A very large corpus was used here (the Web, via queries to a search engine), which appears to be important in making this simple technique work.

Brief description of the method

The algorithm takes a written review as an input. First it assigns a POS tag to each word in the review to identify adjective or adverb phrases in the input review. They have used PMI-IR algorithm to estimate the semantic orientation of a phrase. The Pointwise Mutual Information (PMI) between two words $w_{1}$ and $w_{2}$ is defined as follow:

$PMI(w_{1},w_{2})=log_{2}(p(w_{1}\ and\ w_{2})/p(w_{1})p(w_{2}))$

where $p(w_{1},w_{2})$ is the probability that $w_{1}$ and $w_{2}$ co-occur. They have defined the semantic orientation of a phrase as follow:

$SO(phrase)=PMI(phrase,'excellent')-PMI(phrase,'poor')$

We can modify the above definition to obtain the following formula:

$SO(phrase)=log_{2}({\frac {hits(phrase\ NEAR\ 'excellent')hits('excellent')}{hits(phrase\ NEAR\ 'poor')hits('excellent')}})$

where operator NEAR means that the two phrases should be appeared close to each other in the corpus. Using the above formula they have calculated the average semantic orientation for a review. They have shown that the value of average semantic orientation for phrases in the items that are tagged as "recommended" by the users are usually positive and those that are tagged as "not recommended" are usually negative.

Experimental Result

This approach was fairly successful on a range of review-classification tasks: it achieved accuracy of between 65% and 85% in predicting an author-assigned "recommended" flag for Epinions ratings for eight diverse products, ranging from cars to movies. Many later writers used several key ideas from the paper, including: treating polarity prediction as a document-classification problem; classifying documents based on likely-to-be-informative phrases; and using unsupervised or semi-supervised learning methods.

Related papers

The widely cited Pang et al EMNLP 2002 paper was influenced by this paper - but considers supervised learning techniques. The choice of movie reviews as the domain was suggested by the (relatively) poor performance of Turney's method on movies.

An interesting follow-up paper is Turney and Littman, TOIS 2003 which focuses on evaluation of the technique of using PMI for predicting the semantic orientation of words.