Abstract
This research was motivated by the limitations of assessment instruments capable of measuring students' initial conceptual understanding of mathematics related to real numbers, which is the basis of calculus. This study aimed to develop a valid, practical, and effective diagnostic assessment to measure students' conceptual understanding of mathematics. This research is developmental research with the Plomp model, which includes the stages of preliminary research, development or prototyping, and assessment of the product. Data were collected through interviews, observations, questionnaires, and tests and analyzed using quantitative descriptive analysis. The results of the validator assessment on the validation sheet indicate that the developed diagnostic assessment has a validity level of 91.8% (content aspect), 87.6% (construct aspect), and 92.6% (linguistic aspects). The practicality level was 70.85% (practical), as obtained from the questionnaire scores completed by the students. In terms of effectiveness, the developed assessment met the effectiveness criteria because it could identify areas of conceptual difficulty, differentiate levels of conceptual understanding, and provide meaningful information for decision-making. Its uniqueness lies in the development of a diagnostic assessment item design framework based on specific conceptual understanding indicators for real numbers in higher education and its assessment method.