Data are presented as n (%). NHANES: National Health and Nutrition Examination Survey; ATS: American Thoracic Society; ERS: European Respiratory Society; FVC: forced vital capacity. ^#: in addition to the 348 subjects excluded from the original NHANES reference dataset due to <2 acceptable blows, a further 222 had to be excluded on re-analysis, either because the original flow–volume curves could no longer be retrieved or because best FVC varied by >300 mL from the next best. ^¶: for the purpose of this analysis, the number of acceptable blows ignores the ATS/ERS statement regarding the need for EOT criteria to be met before blows can be considered “acceptable” unless specified by “other criteria”. For example, of the 4568 subjects aged >19 years who were assessed, 4169 (91.3%) subjects achieved at least two repeatable FVC (irrespective of EOT criteria) whereas only 3411 (85.4%) of these subjects also achieved a plateau and an expiratory time of at least 6 s. ⁺: n=4568. ^§: n=3209. ^ƒ: test must have two acceptable blows before any criteria for a repeatable test can be calculated. ^##: 6 s exhalation was determined using total expiratory time or the time that the subject continued exhalation regardless of whether a plateau was achieved. ^¶¶: time required to reach a plateau with ≤25 mL volume change was ≥6 s, i.e. in 54% and 74% of subjects aged ≥19 years and <19 years, respectively, the actual expiratory time required to reach a plateau (defined as a volume change <25 mL in 1 s) was <6 s.