| Level I | Good quality evidence |
Systematic review of RCTs with consistent findings High quality individual RCT |
| Level II |
Limited quality patient orientated evidence |
Systematic review of lower quality studies or studies with inconsistent findings Low quality clinical trial Cohort studies Case-control studies |
| Level III | Other |
Consensus guidelines, extrapolations from bench research, usual practice, opinion, disease-oriented evidence (intermediate or physiologic outcomes only), or case series |
[25] Combined oral contraceptive (containing ethinylestradiol 30-35 mcg) for breakthrough bleeding on progestin-only injectable contraceptives: Faculty of Sexual & Reproductive Healthcare Clinical Guidance: Problematic Bleeding with Hormonal Contraception Clinical Effectiveness Unit, July 2015. https://www.fsrh.org/standards-and-guidance/fsrh-guidelines-and-statements/management-of-srh-issues/problematic-bleeding/