Prospect–Regret Theory-Based MCDM Method Integrating Novel Score Function and Similarity Measure for q-Rung Orthopair Fuzzy Z-Number in Medical Diagnosis

To address the impact of uncertain information reliability in decision-making environments and decision-makers’ psychology on decision results, this paper proposes a multi-criteria decision-making (MCDM) method for q-rung orthopair fuzzy Z-number (qROFZN) based on prospect–regret theory. First, a novel score function for qROFZN is developed, accompanied by a comprehensive discussion of its fundamental properties. Second, the Jaccard similarity distance measure is innovatively proposed for qROFZN, along with an analysis of its essential characteristics. Subsequently, on this basis, the prospect–regret theory is extended to the qROFZN environment, and a novel MCDM method is established. In this method, the criteria importance through inter-criteria correlation (CRITIC) weighting method and prospect theory’s weighting function are employed to determine comprehensive weights, which are then integrated with prospect–regret theory to identify the optimal solution, thereby considering both subjective and objective factors and enhancing the practical relevance of the decision results. Finally, a case study on medical diagnosis is conducted, along with comparative analyses. The experimental results are consistent with the outcomes of the referenced case, which validates the practicality and effectiveness of the proposed method, demonstrating that the qROFZN-based MCDM method can effectively capture fuzziness and reliability, thus enabling accurate decision analysis under uncertainty.