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:Prolonged imbalance of systemic hemodynamics and insufficient postprandial sympathetic activation might cause PPH (Jansen and Lipsitz 1995; Umehara et al. 2014; Hirayama et al. 1993; Mathias 1991).
Category: