The structure and processing in two formally equivalent compound tasks are investigated: a judgment of ease task, and a similarity task. The judgment of ease task is a binary comparison task where two successive binary categorizations are performed followed by a comparative judgment of ease or difficulty of the binary component categorizations. In the similarity-dissimilarity task, the subject must perform two successive binary comparisons each requiring the subject to compare the lengths of a horizontal line segment with a vertical line segment. The second comparative judgment is then immediately followed by the judgment of similarity or dissimilarity of the horizontal segments with the vertical standard.
It is determined that the data can be represented in terms of Coombs' unidimensional theory of unfolding (1950, 1964). For example, when subjects are required to indicate which of two successively presented visual extents is easier or harder to categorize as "long" or "short" they execute these categorizations and then measure the distance of the representation of each stimulus from the long-short category boundary; the stimulus nearer the boundary is the more difficult. When they are requested to indicate which is easier to categorize, they select the alternative that is farther from the boundary.
A model based on inferential rules is advanced as a possible basis for the development of parallel processing. Although no clear evidence of parallel processing was obtained, there was some indication of concurrent processing. The inferential model was supported by obtaining presentation order effects when no explicit perceptual comparisons were required of the subjects.