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:…Van den Heede et al., 2007), others prefer a nurse-reported measure of nurse-to-patient ratio (i.e., number of patients assigned to a nurse during the most recent shift) due to known reliability and validity issues of staffing measures derived from administrative databases (Aiken et al., 2002).
Category: