The paper consists of three main part. The first part is the literature review, talking about the cohesion study by foreign and domestic linguisticians. The following part is the research methodology, showing how the data is collected. The last part is the analysis to the statistics and discussion about the phenomena. 
The purpose of this paper is to find out the differences among students from all grades of their writing skill on lexical repetition/cohesion and to contribute to the improvement of lexical repetition/cohesion teaching in English writing.
2 Literature Review
2.1 Previous Study of Lexical Cohesion in English
    Since Lexical Cohesion in English (2001) has been published by Halliday and Hasan in 1976, people started paying attention to textual lexical cohesion research. In their trailblazing work, Halliday and Hasan defined lexical cohesion as “relations of meaning that exist within text, and that define it as a text” (Halliday & Hasan, 2001, P. 4) and pided it into five types: reference, substitution, ellipsis, conjunction and lexical cohesion. Based on Halliday and Hasan’s theory, Hoey has created his own theory about lexical repetition which is the same as their lexical cohesion. The details of Hoey’s theory will be discussed in the following part.
    Domestic scholars have also made great contribution to the development of lexical cohesion study. Hu Zhuanglin(1994) had put forward the idea the lexical cohesion and coherence is multilevel and he pointed out that phonology as well have lexical cohesion function in English text. He considered that coherence of text must be based on some of the levels. Zhang Delu(1999; 2000) suggest the point that the interpersonal meaning in text also has lexical cohesion function, besides, cohesion can be established through multiple forms and mechanisms. For example,
(1) A: Can you tell me the time?
   B: Well, the mailman has come.
Although the lexical cohesion tie of this example is none of the cohesion ties mentioned in Halliday and Hasan’s theory, it still make sense and keep coherent with semantic relation between the time and has come.
 
2.2 Hoey’s Lexical Repetition Theory
    In Michael Hoey’s Patterns of Lexis in Text (2000), he classified repetition into simple one and complex one. The former one includes simple lexical repetition and simple paraphrase while the latter one covers complex lexical repetition, complex paraphrase, superordinate, hyponymic and co-reference repetition. In Hoey’s theory, cohesion repetition is the same as lexical cohesion.
    Simple lexical repetition, “which occurs when a lexical item that has already occurred in a text is repeated with no greater alteration than is entirely explicable in terms of a closed grammatical paradigm” (Michael Hoey, 2000, P. 53), is both the simplest form of lexical tie and of repetition. The following example is selected from Patterns of Lexis in Text (2000). 不同年级学生英语议论文写作词汇重复研究(2):http://www.youerw.com/yingyu/lunwen_23665.html