The provision of psychiatric care is characterized by wide variations in resource use. Identifying cost-homogeneous groups of psychiatric patients for the purposes of prospective reimbursement has proved problematic. To establish a basis for prospectively reimbursing psychiatric care, the sources of variation in cost, length of stay, and efficient treatment must be explored and documented more thoroughly. In this Note, the authors suggest variables to be included in a data collection effort designed to analyze the sources of variation in psychiatric care. For each variable, they describe why it might be useful as an explanatory variable and where information to identify the variable can be found in the medical record. They also describe the existing data sources likely to include the explanatory variables of interest.