An affinity diagram is a structured synthesis technique used in qualitative research to organise large volumes of unstructured data into meaningful thematic clusters. Researchers write individual observations, verbatim quotes, or ideas onto separate cards or digital notes, then group them based on shared meaning rather than predetermined categories. The resulting clusters reveal patterns that might otherwise stay buried in raw transcripts. In qualitative research, affinity diagrams are particularly valuable during analysis phases where the volume of participant data makes manual review impractical. The method supports grounded, bottom-up thematic analysis, ensuring that the structure of findings reflects what participants actually said rather than what researchers expected to find.