While frequency of occurrence and lexical diversity are common measures of lexical proficiency, very few studies have examined range along with these two other lexical indices, especially in EFL contexts. Using a sample of 529 undergraduate EFL learners' essays, the present study examined the development of three indices of lexical proficiency (word frequency, range and lexical diversity) across 3 different proficiency levels. No similar study has been conducted so far in the Moroccan context,

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... hich makes this study a valuable contribution to a better understanding of university learners' productive lexical knowledge. Unexpectedly, although frequency is an established measure which can successfully show differences in learners' proficiency, the results of this study indicate that at the productive level, the range of content words and lexical diversity might better reflect differences among learners of different proficiency levels. While word frequency was stable among the three groups, indices of range and lexical diversity reflected better lexical knowledge in higher proficiency levels. We concluded that in EFL contexts, range and diversity might be better measures of proficiency. This suggests that there should be more focus on developing these two aspects in EFL classes.