SOURCES OF VARIATION IN SPIROMETRIC MEASUREMENTS - IDENTIFYING THE SIGNAL AND DEALING WITH NOISE

When, in 1848, Hutchinson60 first introduced the term vital capacity ( to comprise ''complemental, breathing and reserve air, conjointly for convenience''), he also proposed its measurement by the spirometer as ''a precise and easy method of detecting disease,'' in his day primari ly phthisis. He also introduced the concept of signal and noise by poi nting out that vital capacity was ''disturbed directly or modified by four circumstances-height, weight, age and disease'' and that to asses s the impact of the latter, the first three needed to be taken into ac count; this, he suggested, could be done by tables he developed for th e purpose based on data-collected on 2,130 subjects. Implicit though n ot explicit in his treatise was the concept that trade or occupation i nfluenced vital capacity, since his study population included sailors in the merchant service, fire brigade men, police, artisans, serviceme n (grenadier guards, house guards, marines), printers, and draymen. He did not elaborate in the text on whether he expected the association to be positive (on the basis of health selection of the fit into their occupation) or negative (on the basis of ill health effects associate d with their occupation). Finally, Hutchinson emphasized the importanc e of standardizing the procedure: he advised three consecutive trials, maintaining the instrument in functioning order, and expressing the r esults at constant temperature (he selected 60-degrees-F).