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:Disease stage was classified according to the International Neuroblastoma Staging System.(15) Survival time was defined as the number of years from diagnosis to death from any cause, or for children who were alive at last follow-up, as the number of years from diagnosis to last follow-up.
Category: