This study investigated lexical density and diversity differences in English as L1 vs L2 academic writing of EFL, ESL, and English L1 postgraduate students to compare their lexical proficiency in EFL vs. English L1 academic settings. A corpus of 210 dissertation abstracts was analysed using three natural language processing tools [LCA, TAALED, and Coh-Metrix] where the effects of text length and topic were controlled. In doing so, we examined the relationship between 15 lexical indices and the construct-distinctiveness of lexical density and diversity. The measure-testing process also assesses the effectiveness of each measure in a pair/group of closely-related measures (in terms of the quantification methods) in capturing lexical diversity differences of these texts. This is to obtain a small number of unique measures that capture lexical diversity as an indicator of lexical proficiency and to assist future writing researchers in the measure-selection process in the face of a multitude of available measures. The findings have important implications for writing assessment and research on lexical indicators of writing proficiency, materials development in EFL academic settings especially for thesis/dissertation writing modules, and a possible contribution of ESL academic immersion programmes in approximating English L1 and L2 proficiency.

这项研究调查了英语作为E1，ESL和L1研究生的L1与L2学术写作的英语的词汇密度和多样性差异，以比较他们在EFL与英语L1学术背景下的词汇水平。使用三种自然语言处理工具[LCA，TAALED和Coh-Metrix]分析了210个论文摘要的语料库，其中控制了文本长度和主题的影响。在这样做的过程中，我们检查了15个词法索引与词法密度和多样性的构造区别性之间的关系。量度测试过程还评估一对或一组紧密相关的量度（就量化方法而言）中的每个量度在捕获这些文本的词汇多样性差异方面的有效性。这是为了获得少量独特的量度，这些量度可以捕捉词汇多样性作为词汇熟练程度的指标，并在面对众多可用量度时，协助未来的写作研究人员进行量度选择过程。这些发现对写作评估和写作能力的词汇指标研究，EFL学术环境中的材料开发（尤其是论文/学位论文写作模块）以及ESL学术沉浸计划对英语L1和L2熟练程度的贡献可能具有重要意义。