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:Kinetic analysis
The duration of phase I was estimated from the averaged response profile, as the time at which there was a marked inflection point in the _VO2, _V CO2 and _V E response and change in end tidal oxygen, end tidal carbon dioxide and R from baseline values (Whipp et al. 1982).
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