A new E n score of the proficiency test (PT) is formulated; it is applicable when a correlation exists between the reference and participant's values. Based on the uncertainty propagation rule given in ISO/IEC Guide 98-3 (GUM:1995), the E n score covering the correlation case is newly developed for the PT. The new E n score will be applied in a future PT organized by the Korea Research Institute of Standards and Science (KRISS) dosimetry team. The new E n score will enhance measurement traceability and contribute to improving the quality management system of participants in the KRISS PT by avoiding performance underestimation.