The Effects of Communicative Purposes on Lexicogrammatical Variation in EFL Prepared Public Speech: A Register-Functional Approach

Prepared English public speaking (EPS) tasks are widely used in EFL programs to enhance learners’ oral communication skills. Yet little is known about how the communicative purposes embedded in different speech tasks shape learners’ linguistic choices in prepared spoken discourse. This study adopts a register-functional (RF) approach and key feature (KF) analysis to investigate lexicogrammatical variation across two common EPS tasks: informative and persuasive speeches. The dataset comprises 236 speeches produced by 118 university EFL students, each annotated for 154 lexicogrammatical features using the Biber Tagger. Applying frequency, dispersion, and effect size criteria, 17 distinguishing features were identified—12 associated with informative and five with persuasive tasks. Functional interpretation revealed that informative speeches were characterized by features supporting audience engagement and structured information delivery, whereas persuasive speeches drew more heavily on features conveying stance and evaluation. These findings underscore the central role of communicative purpose in shaping learner discourse and demonstrate the value of corpus-based RF analysis for advancing pedagogy, material development, and assessment in second language speaking.