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:The contraceptive reliability of the levonorgestrel-releasing intrauterine system (LNG-IUS; Mirena®, Bayer HealthCare, Berlin, Germany) is well established, with efficacy comparable to female sterilization [1–3], but with the added advantage of rapid return of fertility upon removal.
Category: