<![CDATA[The persistent challenges in identifying mathematics learning difficulties require diagnostic instruments that are not only valid but also capable of accurately capturing students’ conceptual understanding, reasoning, and confidence. This study aims to compare the Content Validity Index (CVI) and the Fuzzy Delphi Method (FDM) in expert validation of a three-tier diagnostic assessment for mathematics learning difficulties. Theoretically, the study integrates classical content validation theory with fuzzy logic-based expert consensus analysis to strengthen the methodological rigor of instrument development. The research involved five doctoral-level experts who evaluated 21 diagnostic items. The CVI results yielded an excellent S-CVI/Ave value of 0.97, indicating high content validity, while the FDM results confirmed 20 of 21 items met the validity thresholds (d ≤ 0.2, consensus ≥ 75%, A ≥ 0.5). A key finding shows that the CVI provides clear numerical validation, whereas the FDM captures linguistic uncertainty and confidence variations among experts, producing a more comprehensive validation outcome. The integration of both methods offers a robust framework for expert validation that enhances interpretive depth and accuracy. This study contributes to the field of mathematics education by presenting a validated, reliable, and pedagogically meaningful diagnostic instrument for identifying students’ misconceptions and reasoning patterns.]]>
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