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:…within and 15.20% between assay for 1,25-dihydroxyvitamin D. Proficiency testing was conducted with DEQAS, CAP, API, and NIST for 25-hydroxyvitamin D and with DEQAS and CAP for 1,25-dihydroxyvitamin D as described previously (Smith et al. 2005a,b, 2008, 2009a, 2012b; Zwart et al. 2011b, 2013).
Category: