During 1973–1979, the field coalesced around a rigorous latent trait measurement framework, unifying theory with application by separating item difficulty from respondent ability. Methodological breakthroughs introduced conditional maximum-likelihood estimation for item parameters in multiple-choice tests, enabling unbiased, model-based inference and more reliable test development. The era bridged theory and practice by translating item response functions into concrete guidance for test construction, scoring, and interpretation, while beginning to address fairness considerations that would mature into differential item functioning analysis. Historical Significance: The period established item response theory as a coherent, usable framework for educational assessment, cementing latent trait modeling as the core foundation of measurement. It advanced reliability and validity standards through formal modeling and interpretation of test scores, and it set the stage for widespread adoption of model-based approaches across diverse testing contexts. The innovations from this phase provided lasting analytical tools and conceptual foundations that shaped later growth in measurement theory and applied testing.