Specifically, it centers on the following three questions:
1. What are the differences of the lexical repetition/cohesion ties between high-score essays and low-score essays;
2. Whether the lexical cohesion ties are highly related to the qualities of essays in argumentative writings?
3. Which kinds of lexical repetition/cohesion ties are related to the qualities of essays in argumentative writings?
2 Literature Review
2.1  Studies of Lexical Repetition/Cohesion
Cohesion is the grammatical and lexical linking in a text or sentence that keeps a text together and gives it meaning. It is related to the broader concept of coherence. Cohesion is the essential part in English writing. There are two main types of cohesion: grammatical cohesion which is based on structural content, and lexical cohesion which is based on lexical content and background knowledge. In Cohesion in English, Halliday and Hasan gave a clear definition: “the concept of cohesion is a semantic one; it refers to relations of meaning that exist within the text, and that define it as a text (Zhang Delu, 2001, p. 4) .” The two kinds of cohesion methods are both important to the English writing.
English linguisticians Halliday and Hasan (2001) first make systematic researches on English cohesion and put out the cohesion theory. They together wrote a book called Cohesion in English. In this book, they gave a comprehensive description about the cohesion methods in English texts. They classified these methods into two broad kinds. The first one is the grammatical cohesion and the second one is lexical cohesion. The grammatical cohesion method includes reference, substitution, ellipsis and conjunction. The lexical repetition/cohesion includes antonym ,synonym, hyponymy and collocation.
Another English linguistician, Hoey, has also contributed a lot to the research of the cohesion in his book Patterns of Lexis in Text, published in 1991. He thinks that lexical repetition/cohesion is the main method to make a passage more coherent--of all the cohesion methods, only the lexical repetition/cohesion can compose a stable multilevel semantic relation so the lexical cohesion is the most outstanding and important method in all the cohesion methods. It constitutes about 40% in the textual cohesion bonds. Hoey emphasizes this kind of cohesion in the book Patterns of Lexis in Text.
In China, many linguisticians and researchers have also made progress in the lexical cohesion. Mr Hu Zhuanglin (1994) has written a book named Cohesion and Coherence in the text/ discourse. He explains clearly about several genres of the lexical repetition/cohesion. Mr Zhang Delu writes The Development of the Theory of Text Coherence and Cohesion and its Applications. This book introduces the progress in the cohesion and its applications. It introduces the transitivity, referentiality, structural cohesion and logic cohesion (Zhang & Liu, 2003).
Some researchers have put the repetition/cohesion theory into practice. Lu Yujing and Zhang Aiping (2008) have already wrote Research on the Relation Between the Number of Cohesion Methods and the Quality of Essays. It helps teachers a lot when they are teaching how to write essays. It suggests that teachers should enhance their teaching in lexical cohesion/ repetition and encourage their students to try to use more lexical cohesion/ repetition. Peng Lin and Zhu Pin (2008) have wrote The Applications of Cohesion Methods in the Reading Instruction, which gives us a new angle to do research in the cohesion field. 英语议论文写作中高分作文和低分作文词汇重复手段差异研究(2):http://www.751com.cn/yingyu/lunwen_29788.html