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:Cowan et al. (2004) calculated PD interval by counting HUES-7 periodically over 7 days without passaging, while Amit et al. (2004) achieved efficient separation of H-9 and feeders by exploiting the tolerance of this line to collagenase-passaging on low density MEFs.
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