While there is an abundant literature on the distribution of spirometry statistics in various subsets of the human population, apparently little is known of the structure of the typically very small sample of measures that can be gathered during a spirometry session. This paper starts with a theoretical analysis of the relation linking the measure of forced vital capacity (FVC) to the parameter of total lungs capacity (TLC). Since the maximization effort exerted on FVC measures by the testees is opposed by the resistance of TLC, their impassable personal upper limit, a ceiling effect must take place on the continuum of FVC measurement. Two predictions follow concerning the within-subject distribution of FVC. One is that the distribution should be negatively skewed, the other is that its first and second moments should correlate negatively across sessions. These predictions were tested with the publicly available large-scale spirometry data collected by the Third National Health and Nutrition Examination Survey. Using original data processing techniques especially devised to unveil the shape of small session samples of FVC measures, the paper reports highly consistent confirmatory evidence, based on the analysis of thousands of individual test sessions, that a typical session sample of FVC is indeed strongly skewed negatively and that the session mean and the session standard deviation of FVC do indeed bear a strong negative correlation. Several implications of these results are discussed, some of which cut across the frontiers of respirology. It is suggested that the procedural rigor and simplicity of spirometry testing make it a privileged paradigm for understanding quantitative performance measurement in general.