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 myocardial performance index (MPI) was calculated from the pulsed-wave tissue Doppler tracings, which have been shown to correlate with values from spectral Doppler tracings with similar diagnostic value [5, 10, 11].
Category: