Wordscores, Laver et al. 2003
M. Laver, K. Benoit, and T. College. Extracting policy positions from political texts using words as data. American Political Science Review, 2003.
Abstract from paper
We present a new way of extracting policy positions from political texts that treats texts not as discourses to be understood and interpreted but rather, as data in the form of words. We compare this approach to previous methods of text analysis and use it to replicate published estimates of the policy positions of political parties in Britain and Ireland, on both economic and social policy dimensions. We “export” the method to a non-English-language environment, analyzing the policy positions of German parties, including the PDS as it entered the former West German party system. Finally, we extend its application beyond the analysis of party manifestos, to the estimation of political positions from legislative speeches. Our “language-blind” word scoring technique successfully replicates published policy estimates without the substantial costs of time and labor that these require. Furthermore, unlike in any previous method for extracting policy positions from political texts, we provide uncertainty measures for our estimates, allowing analysts to make informed judgments of the extent to which differences between two estimated policy positions can be viewed as significant or merely as products of measurement error.
This is one of the earliest approaches to estimate the political orientation of text. The political orientation of a text is calculated by scoring every word depending on the probability of word in training documents and the political orientation of these documents.
External link: 
Follow up paper providing a more detailed analysis
- Lowe, Will. 2008. Understanding Wordscores. Political Analysis doi:10.1093/pan/mpn004