| Acknowledgements | 第3-4页 |
| Abstract | 第4-5页 |
| 摘要 | 第6-12页 |
| List of Abbreviations | 第12-13页 |
| Chapter One Introduction | 第13-16页 |
| 1.1 Research Background | 第13-14页 |
| 1.2 Significance of the Study | 第14页 |
| 1.3 Overall Structure of the Thesis | 第14-16页 |
| Chapter Two Literature Review | 第16-33页 |
| 2.1 Key Terms | 第16-19页 |
| 2.1.1 Lexical Richness | 第16-17页 |
| 2.1.2 Lexical Variation | 第17页 |
| 2.1.3 Lexical Density | 第17-18页 |
| 2.1.4 Lexical Sophistication | 第18-19页 |
| 2.2 Theoretical Foundations | 第19-21页 |
| 2.2.1 Interlanguage Theory | 第19-20页 |
| 2.2.2 Output Hypothesis | 第20-21页 |
| 2.3 Previous Studies on Lexical Richness Abroad and in China | 第21-32页 |
| 2.3.1 Studies on Lexical Richness in L2 Writings Abroad | 第21-27页 |
| 2.3.2 Studies on Lexical Richness in L2 Writings in China | 第27-32页 |
| 2.4 Summary | 第32-33页 |
| Chapter Three Methodology | 第33-37页 |
| 3.1 Research Questions | 第33页 |
| 3.2 Research Instruments | 第33-34页 |
| 3.3 Data Collection | 第34-35页 |
| 3.4 Data Analysis | 第35-36页 |
| 3.4.1 Lexical Variation | 第35-36页 |
| 3.4.2 Lexical Density | 第36页 |
| 3.4.3 Lexical Sophistication | 第36页 |
| 3.5 Essay Scoring | 第36-37页 |
| Chapter Four Results and Discussion | 第37-49页 |
| 4.1 Results of Three Dimensions of Lexical Richness | 第37-42页 |
| 4.1.1 Results of Lexical Variation | 第37-39页 |
| 4.1.2 Results of Lexical Density | 第39-40页 |
| 4.1.3 Results of Lexical Sophistication | 第40-42页 |
| 4.2 The Relationship between Lexical Richness and Writing Quality | 第42-46页 |
| 4.2.1 Results of the Writing Quality | 第42页 |
| 4.2.2 Correlation between Lexical Variation and Writing Quality | 第42-43页 |
| 4.2.3 Correlation between Lexical Density and Writing Quality | 第43-44页 |
| 4.2.4 Correlation between Lexical Sophistication and Writing Quality | 第44-45页 |
| 4.2.5 Multiple Regression Analysis of Lexical Richness on Writing Quality | 第45-46页 |
| 4.3 Discussion on Overall Developmental Paths of Lexical Richness | 第46-49页 |
| 4.3.1 The Developmental Path of Lexical Variation across Four Years | 第46-47页 |
| 4.3.2 The Developmental Path of Lexical Density across Four Years | 第47页 |
| 4.3.3 The Developmental Path of Lexical Sophistication across Four Years | 第47-49页 |
| Chapter Five Conclusion | 第49-54页 |
| 5.1 Major Findings | 第49-50页 |
| 5.2 Pedagogical Implications | 第50-52页 |
| 5.3 Limitations and Suggestions for Further Studies | 第52-54页 |
| References | 第54-59页 |
| Appendix Ⅰ: Writing Requirements | 第59-60页 |
| Appendix Ⅱ: Sample Compositions | 第60-64页 |
| Appendix Ⅲ: Marking Criterion for TEM4 Compositions | 第64-65页 |
