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 European Surveillance of Congenital Anomalies (EUROCAT) is a network of population-based registries of congenital anomalies in 21 countries of Europe.(13) The methods of registry case ascertainment are fully described elsewhere.
Category: