Liddell, F. D. K., and Lindars, D. C. (1969).Brit. J. industr. Med.,26, 89-100. An elaboration of the I.L.O. classification of simple pneumoconiosis. Simple pneumoconiosis in chest radiographs presents a continuum of increasing abnormality. Liddell (1963) introduced a 12-point scale obtained by dividing each of the four I.L.O. categories (International Labour Office, 1959) into one central and two marginal zones. In this system, which has come to be known as the N.C.B. elaboration, readers record for each radiograph the I.L.O. category of choice (0, 1, 2 or 3), followed by an adjacent I.L.O. category if that had been seriously considered; otherwise, the same category is repeated. Very clear normals are denoted as 0/-, and `high' category 3 films as 3/4. This paper reviews the evidence from seven reading trials in which 12 National Coal Board (N.C.B.) film readers have taken part. About 28,000 assessments on a total of well over 2,000 single radiographs have been analysed. (The reading of serial radiographs to assess progression is dealt with elsewhere.) All readers used the elaboration successfully, but they differed in the extents to which they placed films in central and in marginal zones; they were more consistent when preliminary briefing had been given. Film quality had little influence on the use of the zones, except that 0/- tended to be reserved for films of good quality. Despite the variation in the use of the zones, marked improvements accrued from the use of the elaboration in both intra- and inter-observer error for all readers, and for films of poor quality as well as for good films. The validity of expressing simple pneumoconiosis prevalence rates in terms of I.L.O. categories derived from N.C.B. elaboration readings was confirmed. Although the exact widths of the zones along the continuum remain to be determined, all the evidence suggests that they represent steadily increasing abnormality. Thus, the N.C.B. elaboration is a practical procedure which amplifies, but neither distorts nor supplants, the I.L.O. classification. It is reported to be easier to use.

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