As with any clinical test, it is important to understand the signal-to-noise characteristics of pulmonary function tests. The signals are the values or the changes in values that are of interest to clinicians and/or investigators. The noise is all the factors that produce variability and can cloud or mask the signal. In pulmonary function testing, the sources of noise can either be technical (eg, instrument, software, technician performance) or biological (eg, activity, endocrinologic state, comorbidities, posture). Low signal-to-noise ratios can come from either a weak signal or overwhelming noise. With low signal-to-noise ratios, separating signal from noise can be very difficult; and depending on the interpretation cutoff chosen, large numbers of false-negative (missed signals) or false-positive (erroneous signals) results can be present.