This study explores coherence as a textual phenomenon in research writing Introductions. Drawing on Systemic Functional Linguistics, the analysis identifies differences in the quality of coherence in both experts' and students' texts by examining the semantic overlaps between the global Themes, followed by the quality of lexical chain deployment, and by how these inform the development of three rhetorical moves. Close analysis was performed on four texts, drawn from the corpora comprising 35 expert and 35 student research Introduction texts. The analysis reveals that the expert and student writers feature different paths in arguing. At global levels, the experts tend to have the key concepts resonate and their arguments are gradually narrowed down to occupy the niche of the study. The key concepts therefore often form into long and mixed chains both to sustain the line of the argument and to elaborate the difficult concepts. In the students' texts, the key concepts are less consistently deployed, evidenced in how different paragraphs can develop different sets of concepts. Their long chains are also often formed by static terms which do not effectively develop the argument. Critical concepts often do not occupy prominent positions or are not mentioned earlier to allow for elaboration. When combined, these features compromised the development of the rhetorical move in the students' texts.
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