Oddsson 2005.

Methods	Randomized trial in 11 countries in Europe and South America.
Participants	1030 healthy women, 18 or more years old Exclusion criteria: contraindication to hormonal contraception, injectable contraceptive use in past 6 months, postpartum or postabortion in past 2 months, breastfeeding in past 2 months, abnormal cervical smear in screening, or drugs that interfere with contraceptive metabolism.
Interventions	Vaginal ring releasing etonogestrel 120 µg + EE 15 µg daily (N=512) versus COC containing levonorgestrel 150 µg + EE 30 µg (N=518); 13 treatment cycles.
Outcomes	Contraceptive efficacy, compliance, safety, cycle control. Subjects used diary cards to record ring or pill use; data used to determine exposure and compliance.
Notes	Corresponding investigator provided bleeding data.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Randomization conducted with interactive voice response system, which gave treatment group and medication number.
Allocation concealment (selection bias)	Low risk	interactive voice response system
Blinding (performance bias and detection bias) All outcomes	High risk	open label
Incomplete outcome data (attrition bias) All outcomes	High risk	Loss after randomization and before treatment: 1079 ‐1030 = 49 Loss after treatment = 298/1030 = 29%; ring 29% and COC 29% Total loss = (49 + 298) /1079 = 32%; ring 33% and COC 31%