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An item response theory is discussed which is based on purely ordinal assumptions about the probabilities that people respond positively to items. It is considered as a natural generalization of both Guttman scaling and classical test theory. A distinction is drawn between construction and evaluation of a test (or scale) on the one hand and the use of a test to measure and make decisions about persons' abilities on the other. Techniques to deal with each of these aspects are described and illustrated with examples.