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:A useful outcome of these results is that despite the higher-thanexpected NAC concentrations, there were minimal adverse effects of the drug seen (Table 2), as previously reported [2–4], although our data demonstrate that adverse effects graded ‘‘mild’’ were more than twice as common in the NAC phase compared with the control.
Category: