17. 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, wherein the association measure is based on a formula selected from the group consisting of Point-wise Mutual Information (PMI), Normalized Point-wise Mutual Information (NPMI), Positive Normalized Point-wise Mutual Information (PNPMI), and Revised Association Score (RAS); creating, with the processing system, a word association profile for the document based on the association measure for each group of content words; and determining, with the processing system, a complexity for the document based on the word association profile, wherein (i) the complexity is used to estimate a reading level of the document or (ii) the document is an essay and the complexity is used to assess a quality of the essay.
17. 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, wherein the association measure is based on a formula selected from the group consisting of Point-wise Mutual Information (PMI), Normalized Point-wise Mutual Information (NPMI), Positive Normalized Point-wise Mutual Information (PNPMI), and Revised Association Score (RAS); creating, with the processing system, a word association profile for the document based on the association measure for each group of content words; and determining, with the processing system, a complexity for the document based on the word association profile, wherein (i) the complexity is used to estimate a reading level of the document or (ii) the document is an essay and the complexity is used to assess a quality of the essay. 18. The computer-implemented method of claim 17 , wherein the word association profile is represented by a histogram.