2024-05-312024-05-312024-05-30https://hdl.handle.net/10125/108165Research into diagnostic assessment of writing has largely ignored how diagnostic feedback information leads to differentiated instruction and learning. This case study research presents a teacher’s account of validating an in-house diagnostic assessment procedure in an English for Academic Purposes writing course with a view to refining it. I developed a validity argument and gathered and interpreted related evidence, focusing on one student’s performance in and perception of the assessment. The analysis revealed that to an extent the absence of proper feedback mechanisms limited the use of the test, somewhat weakened its impact, and reduced the potential for learning. I propose a modification to the assessment procedure involving a sample student feedback report.