This study set out to empirically determine the reliability and validity of the Vocabulary LevelsTests, both the passive and productive versions. Furthermore, attempt was made to investigatethe nature of the students\\\' vocabulary knowledge with regard to their passive and activeknowledge of the L2 words as a whole and at different word frequency levels. Moreover, therelationships between these two types of vocabulary knowledge and the learners\\\' proficiencylevel and reading comprehension ability were studied. And finally, it was scrutinized if therewere any significant differences between the High and Low proficient learners and also Englishmajors and non-majors\\\' passive and active vocabularies. Three tests, the Vocabulary Levels Test,the Productive Version of the Vocabulary Levels Test, and a TOEFL test, were administered to agroup of 76 Iranian undergraduate students majoring in engineering and English Language andLiterature. The results proved the Vocabulary Levels Tests to be reliable and valid tests of vocabulary size. The learners\\\' passive and active vocabularies were also found to be highlycorrelated as a whole and at each separate word-frequency level. Passive vocabulary was alwayslarger than active vocabulary at all levels; however, the gap between the two increased at lowerword-frequency levels. In addition, there was a high correlation between the learners\\\' vocabularyknowledge on the one hand and proficiency and reading comprehension ability on the otherhand. It was also found that there was a statistically significant difference between thevocabulary knowledge of High proficient and Low proficient groups and also between theEnglish majors and non-majors. The High proficient group and the English majors had greaterpassive and active vocabulary knowledge than their corresponding Low proficient group and thenon-majors.

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