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 a study in Vancouver, Brauer et al. (2008) found significant negative associations
between NO2 exposure and fetal growth when they used an AQMS-based approach, but no association when they used a LUR model.
Category: