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:…freely (in order to optimize accuracy of parameter estimates), VO2p data were modeled from the beginning of phase II to 4 min (240 s) of the step-transition; this ensured that each subject had attained a VO2p steady-state, yet did not bias the model fit during the on-transient (Bell et al. 2001b).
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