Factor analysis is useful for examining relationships among large numbers of variables and understanding the conceptual structure of an instrument. The Brief Pain Inventory (BPI), which measures pain in terms of its severity and interference with function, has been widely used with cancer patients. A secondary analysis of BPI data in a sample of postoperative patients is used to illustrate the factor analytic method. The resulting two-factor solution of pain’s interference with function and pain severity are discussed by Margarete Lieb Zalon in relation to the validity of the BPI along with considerations in using the instrument with surgical patients