Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Staffing measures used in previous studies include nurse full-time equivalents
(FTEs), nursing hours per patient day (HPPD), registered nurse (RN) skill mix, nurse-to-patient ratios, and nurseperceived staffing adequacy (Aiken et al., 2002; Schmalenberg and Kramer, 2009; Unruh et al., 2009).
Category: