Because absolute changes in outcomes are difficult to interpret and the minimal clinically important difference (MCID) is not suitable to address this challenge, a novel method of classifying outcomes by relating changes to baseline values is warranted. We used the “performance score” (T2D), which reflects individual performance, enabling us to consider the functional status at the beginning of rehabilitation without dealing with the problems of mathematical coupling or regression effects, as encountered in ANCOVA. To illustrate the T2D, we retrospectively analyzed changes in the six-minute walking test (6MWT) in COPD patients undergoing outpatient pulmonary rehabilitation and compared the results with absolute differences related to a predetermined MCID. We evaluated a total of 575 COPD patients with a mean age of 61.4 ± 9.2 years. 6MWT improved significantly, with a mean change of 32.3 ± 71.2. A total of 105/311 participants who had reached the MCID were still classified as “below average” by the T2D. Conversely, 76/264 patients who had not reached the MCID were classified as “above average”. This new performance measure accounts for the patient’s current status and for changes over time, potentially representing a simple and user-friendly tool that can be used to quantify a patient’s performance and response to rehabilitation.