Vocabulary Mastery and Grammatical Competence as Predictors of EFL Students' Writing Ability: Evidence from an Indonesian Vocational High School

Abstract

Writing remains a challenging skill for learners of English as a foreign language (EFL) in Indonesia because it requires students to combine ideas, vocabulary, grammar, and text organization into coherent writing. This study examines vocabulary mastery and grammatical competence as predictors of the writing ability of grade XI vocational high school students in Jayapura, Papua. This study used a quantitative correlational design. The population consisted of 56 grade XI students at SMK Hikmah, Yapis Jayapura in the 2024/2025 academic year, and 23 students from the Accounting program were selected as the sample through purposive sampling. Data were collected through three tests: a vocabulary mastery test, a grammatical competence test, and a writing ability test using a maze-based writing task. The data were analyzed using descriptive statistics, correlation analysis, and multiple linear regression. The findings show that students' vocabulary mastery, grammatical competence, and writing ability were positively related. The regression model was statistically significant, F(2, 20) = 489.805, p < .001, with R = .990 and R² = .980. Vocabulary mastery (B = .538, p = .001) and grammatical competence (B = .432, p = .004) were both significant predictors of writing ability. These findings show that writing instruction in EFL classes at vocational high schools needs to combine lexical development and communicative grammar practice instead of treating vocabulary and grammar as separate language components. This study provides contextual evidence from an underrepresented EFL setting in eastern Indonesia and offers practical implications for strengthening writing instruction in vocational high schools.