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:Unlike other phenomena showing longer-term memory effects on attentional search (e.g., contextual cuing; Chun & Jiang, 1998, 2003), PoP appears to be a short-term effect that lasts approximately 15–30 sec (Maljkovic & Nakayama, 2000).
Category: