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:…in the tree-cover product was selected with the assumption that pixels with at least 10% tree cover have the potential to be classified as forest under natural conditions, in order to account for the deforestation that has occurred during the last decades (Liu et al., 2001; Viña et al., 2007).
Category: