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:…lower compared to the high mean concentrations of Ni reported in Morocco and India (85 and 66.55 mg/kg, respectively) by Al-Jaboobi et al. (2014) and Yadav et al. (2013), but higher than those from Nigeria and India (0.77 and 1.3 mg/kg, respectively) (Rapheal and Adebayo 2011; Brar et al. 2000).
Category: