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:Similarly, Tilahun et al. (2013) concluded that a combination of manure (15 t ha-1 year-1) and inorganic fertilizer (120 kg N ha-1 year-1 and 100 kg P ha-1 year-1) gives high yield of lowland rice in the Fogera plains of Ethiopia.
Category: