A study has been made in working coal-miners of the repeatability of ventilatory tests, anthropometric measurements, and the answers to a questionnaire on respiratory symptoms.On the first visit especially, there was evidence of a continuous increase in both forced expiratory volume (F.E.V.) and forced vital capacity (F.V.C.) from the first to sixth expirations at the same visit but it was minimal after the first expiration. No difference between the beginning and end of a shift was found. The effect on the F.E.V. and F.V.C. readings of the day of the week and time of day at which the examination was made was negligible. The observations recorded in Scotland were 15 % higher than those obtained in South Wales. Between the beginning and end of shift there was a small reduction in weight and in standing and sitting height measurements.The repeatability of the answers to the questionnaire was good. No consistent difference was found between doctor and clerk observers.In 1956 it was decided to include follow-up studies of ventilatory function as part of the medical examinations carried out by the Pneumoconiosis Field Research of the National Coal Board (Fay, 1957; Fay and Rae, 1959). In routine surveys in this scheme the mineworkers are all volunteers. This implies that they may present themselves for examination on any day of the week and either before or after working a shift. It was therefore considered necessary to examine the reproducibility of the proposed measurements. Investigations were carried out at a colliery in Fife and a colliery in South Wales before the routine surveys were started, to study the following factors:-(1) The variability in the measurements of the one-second forced expiratory volume (F.E.V.) and forced vital capacity (F.V.C.) at the same visit; (2) the effect of working a shift; (3) the effect of the time of day at which the man was examined; (4) the effect of the day of the week on which the man was examined; (5) the observer error when making measurements of F.E.V., F.V.C., standing height, sitting height, and weight; (6) the reproducibility of a short questionnaire on respiratory symptoms. DesignThe design of the trial was conditioned by the need to investigate the effect of the day of the week and the time of day at which the various tests were applied. As it was thought that the age and occupation of the subject might also have some effect on the results, account was also taken of these factors. At each colliery the working population was divided into five arbitrary age classes (under 30 years, 31-40, 41-50, 51-60, and 60 and over) and five arbitrary occupational classes, of which two covered faceworkers, two covered non-faceworkers underground, and one covered surface workers. Each of these 25 groups was further subdivided into three subgroups, in terms of shift worked (days, afternoons, or nights) on the first of the two weeks during which the trial was carried out. The subjects for examination were then chosen at random, one from each of the 75 subgroups. To obtain a balance...

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