As part of the redesign of the Common Core of Data of the National Center for Education Statistics (NCES), and in response to the growing concern about the comparability of average daily attendance (ADA) in a national database, the NCES initiated a study in 1987 to describe the methods used by the states to collect, aggregate, and report state total ADA. Five research questions were addressed: (1) whether or not ADA is defined by state law; (2) the number of days in the school year used in the ADA calculation; (3) how attendance is counted and how state aggregate data are calculated; (4) whether or not other variations in state ADA affect comparability; and (5) how summer school figures are handled. Data for the study were derived from state-provided documentation. Seven states reported legislation prescribing data collection methods for the 1985-86 school year. A variety of prescribed school days (ranging from 170 to 180) were identified. Five distinct patterns for state data collection and aggregation ADAs were apparent. Other variations in procedures did affect comparability. However, it was not possible to estimate the effects of summer school attendance reporting, with only 13 states reporting summer school data. Four tables contain study data. (SLD)