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:All data were analysed using general linear models implemented in the R statistical 228 package (R 2005), and for all models, the significance of the predictor variables was 229 examined hierarchically using a stepwise backward model reduction procedure 230 (Crawley 2007).
Category: