Figure 2 Diagrammatic representation of the relationship between the average biomarker concentration and severity of lung disease. The data are related to three populations of individuals at no risk with low levels of the biomarker, those at intermediate risk with intermediate levels and those at high risk with high levels. In cross‐sectional studies, as the disease severity increases the proportion of patients in the high‐risk group also increases, leading to a rise in the average biomarker level for the group.