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 primary studies investigated BNP or NT-proBNP in intensive care patients who were critically ill for use in diagnosis (7, 9), prognosis (6, 8, 11, 12) or monitoring (10).
Category: