Theories and measurement in disability.

of disability was described which gave rise to an appropriate measurement technique. The model was presented as 'an intuitive construct or concept of disability', and attention was concentrated on validating the measurement technique in terms of it. The present paper, by contrast, attempts to draw out the theoretical assumptions of that intuitive construct, and to compare them with two other theoretical positions. The aim is, firstly, to develop and demonstrate the explanatory power of the model, and, secondly, to derive from its explanatory principles further implications for the measurement of disability. It will be evident that the original conception and the first results of the scaling technique already published have had a reciprocal influence on one another, and the theoretical development is not easily characterised as either deductive or inductive, but it is presented in a quasi-deductive form in order to test it against fresh data and analysis. The scaling technique described in- the earlier paper had both strengths and weaknesses. Its strengths lay in its objectivity, and in the very minimal assumptions it required. It concerned the way in which disabled people demonstrably behaved, yet in most cases their patterns of behaviour turned out to be such that uncontentious judgements of disadvantage could be made. Moreover, the scale had the unusual virtue of detecting those individuals to whom its assumptions did not apply. The weaknesses were in part those of the modest level of rank-order measurement achieved. Certain questions still had to be left to arbitrary judgement, including the relative ranking of differences between ranks in the scale. An interval scale would probably be needed for this task. There remained also the question of how to assign cases to whom the scale assumption did not apply. Finally, it was not clear how the theoretical model would determine the relevant universe of disability items. This paper attempts to make progress in these areas. It is usual to distinguish between impairment, disability, and handicap, but there is an intermediate area which requires the distinction of a fourth