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:Nowadays, measurement of serum CA125 is considered essential for monitoring response to treatment (Van der Berg et al, 1988; Tuxen et al, 1995; Rustin et al, 1996) but has a limited value for general population screening (Tuxen et al, 1995; Zanotti and Kennedy, 1999).
Category: