1. 研究目的与意义（文献综述包含参考文献）

2. Literature ReviewResearchers in the areas of applied linguistics and instructed second language acquisition have always been interested in measuring second language performance. One shortcoming of the research done so far on task performance is that it has focused mostly on the syntactic aspect of complexity, with very few studies investigating the lexical aspect of this performance area. Skehan (2009b) states that lexis has been strikingly absent in task research and that it is vital to incorporate some measures of lexis into task performance. The three dimensions of complexity, accuracy, and fluency thus need to be supplemented by measures of lexical performance. The range of measures also needs to be widened to cover this additional area. Most of the studies conducted in the area of task performance have used only lexical diversity as the measure of lexical performance. We also need to consider how different measures of lexical performance correlate and how the lexical measures relate to other measures whether, for example, they relate to syntactic complexity, accuracy, or neither (Skehan, 2009a). 2.1 Basic concepts2.1.1 Productive VocabularyThe range, variety, or diversity of words found in learners language use is believed to reflflect the complexity of their vocabulary knowledge as well as the level of their language profificiency. Nation (1990) pointed out that receptive vocabulary is the examination of vocabulary knowledge from the perspective of language comprehension, which refers to the vocabulary that learners can understand in reading or listening discourse; productive vocabulary is the examination of vocabulary knowledge from the perspective of language use, which refers to learning Vocabulary that the writer can express freely in writing and speaking.2.1.2 Lexical Diversity lexical diversity is the variety of active vocabulary deployed by a speaker or writer(Malvern and Richards,2002: 87). Lexical diversity is an example of text-internal measures, which is typically measured through some sort of type-token ratio (TTR). A serious problem with TTR measures is that they are affected by text-length or sample size and a correction has to be made (Malvern Richards McCarthy D provides an index of the extent to which the speaker avoids the recycling of the same set of words. If a text has a lower D, it suggests that the person producing the (spoken or written) text is more reliant on a set of words to which he or she returns often. (Skehan, 2009a, p. 108).Lexical diversity has been considered an illuminative predictor of learnersgeneral language proficiency (e.g. Zareva et al. 2005) and an essential indicator of the quality of their writing (e.g. Laufer and Nation 1995) and speaking (e.g. OLoughlin 1995; Jarvis 2002; Malvern and Richards 2002) performances. 2.1.3 Lexical SophisticationIn contrast, measures of what is called lexical sophistication (Read, 2000) take frequency lists from corpus analysis and then compute how many words defined as difficult are used in a text, with difficulty being defined on the basis of lower frequencies. Laufer and Nations (1999) Lexical Frequency Profile is the most well-known measure of this sort. The profile provides information on the number of words in a text drawn from the 1000 word levels, the number drawn from the 2000 word levels, and so on. It enables a judgement to be made regarding the extent to which very frequent words are relied upon less. An alternative measure is P_Lex developed by Meara and Bell (2001), which uses a mathematical modelling procedure. It divides a text into ten-word chunks and computes the number of infrequent words in each ten-word chunk. The random sampling method can be used to analyze shorter texts and compare texts of different lengths, so it is more suitable for analyzing the productive texts of second language learners(Bao Gui, 2013).2.2 Related theories and empirical studiesBrumfit (1979) proposed accuracy on the one hand and fluency on the other hand as two important aspects of language use. Skehan (1998) added complexity as another important aspect of language use, and thereby the triad of complexity, accuracy, and fluency (CAF) were introduced as the three fundamental dimensions that characterize second language performance. These three components have proved useful measures of second language performance (Skehan, 2009b).Ellis (2003) offers the following working definitions for the three dimensions. Complexity refers to the extent to which the language produced by the learners is elaborate and varied. It is divided into syntactic and lexical complexity. Accuracy is defined as the extent to which the language produced by the learner conforms with target language norms. Fluency refers to the extent to which the language produced by the learner manifests pausing, hesitation, or reformulation. Skehan (2009b) characterizes successful task-based performance as containing more advanced language, leading to complexity; a concern to avoid errors, leading to higher accuracy if this is achieved; and the capacity to produce speech at normal rate and without interruption, resulting in greater fluency (p. 510).Using data from six studies, Skehan (2009a) conducted a meta-analysis on the relationship between lexical diversity and lexical sophistication and the relationship between these two aspects of lexical complexity and other aspects of performance, such as syntactic complexity, accuracy, and fluency. The six studies used a range of task types and task characteristics, falling into one of three categories: personal information exchange; narratives, either based on picture stories or on a video; and decision-making, where students were required to make decisions. The six studies used to form the basis for the meta-analysis were Foster and Skehan (1996, 2013), Skehan and Foster (1997, 1999, 2015), and Foster (2001). The studies used D as the measure of lexical diversity and lambda as the measure of lexical sophistication. Syntactic complexity was operationalized as the mean number of clauses per ASU, which is an index of subordination.According to Gong Fangfang(2007), Chinese ESL learners do not demonstrate an increase in the use of different words in their oral work with the year of learning. Such an explanation is contrary to the researchers presumption that L2 learners lexical variety grows with the years of learning. However, the lack of a difference in lexical variation between the two groups of students may be due to the sensitivity of the measure of lexical variation to text type and content.In a longitudinal case study, Kalantari and Gholami (2017) explored Iranian EFL learners lexical complexity development over a period of six months in the essays written by five intermediate to advanced EFL learners. They also investigated the correlation among lexical complexity indices. The results indicated that there was a positive correlation between lexical density and lexical sophistication. Lexical diversity, however, did not correlate significantly with both lexical density and lexical sophistication.The main purpose of the study was to answer the following questions:1. Does lexical diversity change with the proficiency level of ESL learners? If so, what is the difference between high- and low-level groups?2. Does lexical sophistication change with the proficiency level of ESL learners? If so, what is the difference between high- and low-level groups?

2. 研究的基本内容、问题解决措施及方案

3.Methodology3.1 Research question1. Does lexical diversity change with the proficiency level of ESL learners? If so, what is the difference between high- and low-level groups?2. Does lexical sophistication change with the proficiency level of ESL learners? If so, what is the difference between high- and low-level groups?3.2 Research designThis study adopts a quantitative method to investigate the difference of lexical diversity and lexical sophistication beween high- and low-level groups. 3.2.1 ParticipantsStudents majoring in English in Nanjing Tech University are subjects in this experiment. All of them, totaling 14 from English majors.3.2.2 Research tools and data collection In this study, oral recording was used, and all oral output was recorded by the researcherin the oral classroom, and then sorted into written text. This research test is divided into two parts: oral composition and comments. Under the task, the test subjects have 5 minutes of preparation time. In addition, there are no unnecessary restrictions. The testees can give full play to the class. The classroom atmosphere is relaxed, and the oral output can reflect the testee's true level.3.3 Analytical method Lexical diversity was expressed by the Mean Type/Token Ratio; Lexical sophistication was represented by the Lexical Frequency Profile and the Condensed Lexical Frequency Profile. Independent-Sample T test is adopted to compare the differences between the two English proficiency groups in these two measures.