The ZPG Letter: Subject, Definiteness, and Information-status – Prince (1998)
Prince gives an analysis of Subject/Nonsubject NPs in a letter. She annotates the letter for Hearer Old/New, Discourse Old/New, Inferable, Pronominal/Nonpronominal, and Definiteness. She shows that the only reliable predictor of Subject/Nonsubject was Discourse Old/New. She argues we could count saliency as another factor if we take the distinction between Pronominal/Nonpronominal NPs to be representative of salience. All this research claims is that the letter conforms to genre tendencies for Subjects to be old information, specifically discourse old information. It would be nice to see this work extended to a large dataset so generalizations could be made.