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:…I analyzed these data using a multi-level mixed-effects linear regression (GLMM, lme package in R software, 2.7.2,
Foundation for Statistical Computing, R Development Core Team 2008), which uses t-tests to test the null hypothesis that a coefficient equals 0 (Bolker et al. 2009; Crawley 2007).
Category: