Fig. 1From: Biological quality control for cardiopulmonary exercise testing in multicenter clinical trialsEffect of z-score on normality of oxygen uptake (V̇O2) measurement distribution. Normalcy of biological quality control test measurements selected on the basis of a composite z-score. Above a critical z-score of 0.67, the W statistic drops dramatically and the distribution of paired differences becomes non-normal for V̇O2 at 70 WBack to article page