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:The average rate at which these radioactive boluses were transported along the trachea by mucociliary activity was measured by using six aligned 8-mm-thick scintillation detectors in conjunction with a six-channel count/time recorder, as previously described (31, 38).
Category: