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:The presence of severe exacerbation in the previous year is a risk factor for mortality, which confirms the results from other studies (Almagro et al., 2002;Kim, Clark & Camargo, 2006; Piquet et al., 2013; Müllerova et al., 2015).
Category: