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:However, our previous data on healthy paediatric patients (age 3 to 18 years) undergoing catheterisation for atrial septal defect closure resulted in a transfer function with the same peak at the fourth harmonic as demonstrated by Nichols and O’Rourke [8] suggesting the validity of this technique [9].
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