<![CDATA[Pharmaceutical calculations require a solid understanding of ratios, proportions, and percentages to ensure accurate drug preparation and dosage determination. However, pharmacy students often struggle to apply these concepts, which may lead to misconceptions. This study aimed to develop a modified three-tier diagnostic test (MTTDT) to identify pharmacy students' misconceptions in ratio, proportion, and percentage calculations. The instrument was developed using the ADDIE model. Unlike conventional three-tier tests, the second tier was modified from a multiple-choice to an open-ended essay format, enabling separate assessment of conceptual understanding, problem-solving process, and calculation accuracy. Content validity was evaluated by seven experts using Aiken's V, while empirical validity and reliability were assessed in a separate validation cohort using point-biserial correlation and Cronbach's alpha (α = 0.727). Of the ten items initially designed, nine met the empirical validity criterion and were retained for the main study, which was administered to 97 first-year pharmacy students. On average, 20.44% of responses reflected misconceptions and 25.44% reflected a lack of knowledge. Analysis of the open-ended second-tier responses revealed a marked disparity between conceptual understanding (81.56%) and calculation accuracy (43.00%), although most students recognized relevant concepts; their performance declined substantially during the problem-solving and unit-conversion steps. These findings suggest that the modified three-tier diagnostic test, particularly its open-ended second tier, can effectively surface procedural and unit-conversion-related misconceptions that conventional multiple-choice formats may fail to detect, supporting its use for targeted instructional intervention in pharmaceutical mathematics education.]]>
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