During the first seven years of the research effort on high-temperature superconductors, the average publication rate has been approximately 5000 papers per year. As may be expected, this published literature has an abundance of seemingly conflicting results, and there are major concerns about the consistency and the comparability of the reported property values. This paper discusses an evaluation methodology that addresses those issues using a structured evaluation procedure. The procedure applies to each set of data and leads to the assignment of one of seven well-defined data evaluation levels. The general procedure and criteria are discussed first, and then special relations that may result in data of higher quality are discussed. The latter relations consider the roles of material structure, composition, energy levels, and materials property relations. The methodology is illustrated by an evaluation of data for YBa²Cu³O^x for which there is an especially important dependence on composition and crystallographic structure.