Refined item digraph analysis (RIDA) is an approach to the study of diagnostic and prescriptive testing. It permits the assessment of the diagnostic value of a test item by examining the extent to which test items have the properties of an ideal diagnostic test item. The first two properties of an ideal diagnostic test item and their quantitative indices are described and illustrated in an analysis of the items in the Orange Juice Test (OJT) assessing understanding of the concept of proportionality. © 1990, Taylor & Francis Group, LLC. All rights reserved.