Fig. 3From: Biological quality control for cardiopulmonary exercise testing in multicenter clinical trialsDistribution of relative frequencies of paired differences from zero. Differences from zero for oxygen uptake (V̇O2), carbon dioxide output (V̇CO2), and minute ventilation (V̇E) at rest (a–c), 20 W (d–f), and 70 W (g–i). None of the variables show normal distribution when all tests are considered (Shapiro–Wilk test, P < 0.001). Test selection based on z ≤ 0.67 resulted in all data at each level of exercise (with the exception of V̇E at rest) being normally distributed (Shapiro–Wilk test, P > 0.05)Back to article page