Test reproducibility is an important consideration when interpreting results and should be set as a goal during data collection. Reproducibility criteria may need to be different for different subject groups and are instrument and procedure-dependent. Ideally, the within-subject variability for each lung volume and measurement technique used should be established for each laboratory. These values also need to be established for each different subject group (age and disease). At a minimum, test reproducibility should be monitored and controlled and each laboratory should define their between-day reproducibility of measurements on at least one "reference" subject from ongoing periodic (e.g., weekly or monthly) measurements as part of their laboratory's quality control programme. For plethysmographic measurements functional residual capacity (FRC)pleth multiple determinations and a corresponding test reproducibility criteria is probably justified.