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:Patients were also stratified (as best as possible) using the available ACSNSQIP variables into the VHWG defined grading system [3], Modified VHWG grading system [16], Ventral Hernia Risk Score (VHRS) [11] and National Nosocomial Infection Index (NNIS) [17] (Table 1).
Category: