Data aggregation: criteria for psychometric evaluation.

Criteria for the examination of data collected at the individual level and aggregated to the group are proposed. These four empirical criteria relate to content validity, representativeness, reliability, and validity of the aggregated scores as indicators of group phenomena. An example of the use of the criteria is provided using data from a large research project on the implementation of a professional practice model on nine nursing units. The use of the criteria proposed provide a systematic approach to the evaluation of individual data for use at the group level.