1. A computer-implemented method of analyzing complexity of a document, comprising: receiving the document with a processing system, the document including a plurality of words; identifying, with the processing system, content words within the document based on parts of speech for the plurality of words; selecting, with the processing system, a plurality of pairs of the content words that appear in a same sentence or paragraph to form multiple groups of content words; determining, with the processing system, an association measure for each group of content words by determining how often the pair of content words in each group appears in a same sentence or paragraph in a corpus of documents; creating, with the processing system, a word association profile, wherein the word association profile comprises a distribution of association measures for the multiple groups of content words in the document; determining, with the processing system, a complexity for the document based on the word association profile, wherein the document is an essay and the complexity is used to assess a quality of the essay.