Many textbooks include semantically related words and sometimes teachers add synonyms, antonyms, etc. to the words in order to present new vocabulary items without questioning the possible effects. This study sought to investigate the effect of teaching vocabulary through synonym, semantically unrelated, and hyponym sets based on Higa’s (1963) proposed continuum. A total of 120 Iranian intermediate EFL adults were selected and classified into two high and low language proficient learners based on their PET (2003) scores. They learned the vocabulary items based on the three above-mentioned methods. Learners’ vocabulary achievement was measured using Paribakht and Wesche’s (1993) Vocabulary Knowledge Scale (VKS) in order to assess both the quantitative (number of learnt vocabulary [NLV]) and the qualitative knowledge of vocabulary (depth of learnt vocabulary [DLV]) by administering the same test twice with a two-week interval for obtaining ST and LT results. To address research questions, two independent two-way ANOVAs and two mixed design two-way ANOVAs were conducted. The results revealed that the learners from synonym sets group gained better ST vocabulary achievement quantitatively and language proficiency level proved not to play any significant role in the learners’ vocabulary accomplishment based on belonging to any given group. It was also revealed that quantitatively hyponym, semantically unrelated, and synonym set groups were respectively the most effective methods of clustering that lead to less forgetting in LT which supports Higa’s proposition.
