Algorithm to determine how positive or negative a statement/text is

Question

I need an algorithm to determine if a sentence, paragraph or article is negative or positive in tone... or better yet, how negative or positive.

For instance:

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Answer 1

Ah, I remember one java library for this called LingPipe (commercial license) that we evaluated. It would work fine for the example corpus that is available at the site, but for real data it sucks pretty bad.

Answer 2

There is a sub-field of natural language processing called sentiment analysis that deals specifically with this problem domain. There is a fair amount of commercial work done in the area because consumer products are so heavily reviewed in online user forums (ugc or user-generated-content). There is also a prototype platform for text analytics called GATE from the university of sheffield, and a python project called nltk. Both are considered flexible, but not very high performance. One or the other might be good for working out your own ideas.