There is a need to assess the accuracy of CGM systems for several uses. Mean absolute relative difference (MARD) is the most common measure for this purpose. Unfortunately, it is frequently overlooked that MARD values computed with data acquired during clinical studies do not reflect the accuracy of the CGM system alone, but are strongly influenced by the design of the study. Among others, the glucose range has a significant impact on CGM performance with typically lower performance in low and possibly also high glucose ranges. Therefore, the percentage of reference measurements in those ranges has a strong impact on MARD.