摘要：EFL learners’ ability to use advanced and varied vocabulary is a crucial issue in writing performance. This study aimed to investigate the effect of writing task type on lexical diversity and sophistication. The relationship between lexical sophistication and lexical diversity in the narrative, argumentative, and descriptive task types written by upper-intermediate EFL learners was also explored. To this end, 70 EFL upper -intermediate learners, enrolled in advanced writing tasks, were selected as the participants. They undertook narrative, descriptive, and argumentative writing tasks in a counterbalanced way. Then, Vocabulary Diversity (Voc-D) and Measure of Textual Lexical Diversity (MTLD) were used to measure lexical diversity. Lexical sophistication was measured using CELEX log frequency and Beyond 2000 Words (K3+). The writings were pasted into Coh-Metrix measure MTLD, Voc-D, and CELEX. VocabProfiler was also used to measure K3+ of the writings. In lieu of the purposes of the study, the research design involved repeated measures and correlation. Using Multivariate Analysis of Variance and Pearson’s Correlation Coefficient, the findings revealed that lexical diversity varied significantly across the three writing tasks. The highest scores of MTLD and Voc-D belonged to the narrative, argumentative, and descriptive writing tasks, respectively. Moreover, EFL learners used more advanced words in argumentative task type according to the measurement of CELEX and K3+ level words. However, learners' lexical sophistication in narrative and descriptive writings did not emerge as significantly different when measured by K3+. The significant positive relationship between lexical sophistication and lexical diversity was evident in all three task types.
