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:Although PPH is one of the most important types of autonomic failure in patients with PD (Jansen and Lipsitz 1995; Loew et al. 1995; Umehara et al. 2014) and related disorders (Hakusui et al.
Category: