Data evaluation is the process by which collections of data are assessed with respect to reliability, completeness, and consistency. The present work addresses data evaluation for materials properties as a scientific discipline that evolves from the formal underpinnings of materials metrology. A theoretical foundation for data evaluation is developed and then illustrated in a practical application to the development of an operational protocol for materials property data. An extensive collection of examples is used to examine, in succession, the issues of accessibility, reproducibility, consistency, and predictability. Distinctions are made among definitive relations, correlations, derived and semiempirical relations, heuristic theories, and value estimates. Subtopics include the use of properties as parameters in models, the interpretation of ad hoc parameters, and the treatments of procedural properties, response dependent properties, and system dependent data.