Davar 2011.

Study characteristics
Methods	Single‐blind randomised clinical trial Duration and location of the trial: quote: "In this single blind randomized trial, 148 ovarian cycles were studied in 100 clomiphene‐ resistance patients with PCOS who were chosen among 250 PCOS patients attending the Research and Clinical Center for Infertility, Shahid Sadoughi University of Medical Sciences, Yazd, Iran during the years 2007‐2008.“
Participants	Inclusion criteria: women who received 150 mg CC daily for 3 cycles and failed to become pregnant, and were diagnosed with anovulatory PCOS based on Rotterdam 2003. Exclusion criteria: quote: “We excluded patients with liver and kidney dysfunction, cardiovascular disease, diabetics, and those who use metformin or drugs affecting insulin secretion and CC in recent 2 cycles.” Number of centres: 1, research and clinical centre for infertility, Shahid Sadoughi University of Medical Sciences, Yazd Number of women randomised: 100 women, 50 in group A metformin‐letrozole, 50 in group B metformin‐CC Number of women analysed: 48 in group A metformin‐letrozole, 50 in group B metformin‐CC Number of withdrawals/exclusions/loss to follow‐up and reasons: 2, experienced side effects with metformin before letrozole was started Age (y): group A metformin‐letrozole: 28.5 ± 3.1, group B metformin‐CC: 29.6 ± 3.5 BMI (kg/m²): group A metformin‐letrozole: 29.0 ± 3.8, group B metformin‐CC: 29.2 ± 2.9 Duration of infertility (y): group A metformin‐letrozole: 3.8, group B metformin‐CC: 3.8 Country: Iran
Interventions	Group A: metformin 1500 mg daily for 6‐8 weeks, followed by 5 mg letrozole daily orally given for 5 days during cycle days 3‐7 if pregnancy did not occur Group B: metformin 1500 mg daily for 6‐8 weeks, followed by 100 mg CC daily orally given for 5 days during cycle days 3‐7 if pregnancy did not occur
Outcomes	E2 (pg/mL) on day of hCG administration, number of follicles > 18 mm in diameter, endometrial thickness on day of hCG administration (mm), clinical pregnancy rate, miscarriage rate (abortion)
Notes	Ethical approval: yes, the trial was approved by ethical board of Shahid Sagoughi University of Medical Sciences, Yazd. Informed consent: no, at least nothing written about it – authors contacted Source of funding: quote: “the study was fully supported and funded by Shahid Sadoughi University of Medical Sciences, Yazd, Iran” Power calculation: quote: "In this study, 50 cases were needed in each group so as to gain a significant difference of 22% in pregnancy rate at a significant level of 5% and a power of 80%" We contacted Dr Davar by email to get additional information, but we did not get a response.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Randomisation was done using a random‐numbers table
Allocation concealment (selection bias)	Unclear risk	Not stated
Blinding of participants and personnel (performance bias) All outcomes	Unclear risk	Not stated who was blinded in this single‐blinded trial
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	Not stated who was blinded in this single‐blinded trial
Incomplete outcome data (attrition bias) All outcomes	Low risk	2 participants lost to follow‐up due to side effects experienced with metformin before letrozole was started.
Selective reporting (reporting bias)	Low risk	All outcomes reported stated in the protocol
Other bias	Low risk	None