Zarei 2015.

Study characteristics
Methods	Randomised controlled clinical trial Duration of the trial: quote: "They underwent intra uterine insemination (IUI) between August 2011 and December 2012.“
Participants	Inclusion criteria: quote: "patients with CC‐resistant PCOS. According to Rotterdam criteria, patients with at least 2 out of 3 of below criteria were included as a PCOS: 1) chronic anovulation, 2) clinical and/or biochemical evidence of hyperandrogenism and 3) polycystic appearance of ovaries in Transvaginal Ultrasound (TVS) Moreover, infertility was defined as failure to conceive despite having unprotected and frequent intercourse for at least 1 year. CC‐resistance was considered as absence of ultrasound evidence regarding ovarian response consumption of 150mg of CC between the 5th and the 9th day of menstruation cycle for three consecutive cycles. All participants had a documented normal blood test, renal function test, liver function test, hysterosalpingography (HSG) and negative pregnancy test before the trial. The partners should have at least two semen analyses. According to WHO, a normal semen analysis should have these properties: Sperm concentration ≥ 15 million/ml, total sperm count ≥ 39 million, mobility rate > 40%, progressive motility ≥ 32% and normal morphology ≥ 4%." Exclusion criteria: women with breast cancer, renal and liver diseases, autoimmune problems and endocrinological problems such as diabetes, hyperprolactinaemia, thyroid diseases, Cushing’s syndrome and smokers were excluded from the trial. Number of women randomised: 140 women Number of women analysed: 131 women were analysed, 67 patients in control group and 64 cases in letrozole group. Number of withdrawals/exclusions/loss to follow‐up and reasons: quote: "during this study, we eliminated 6 patients from the control group, 4 fell out of the study and 2 were finally diagnosed for OHSS. Three patients were also eliminated from the letrozole group; one fell out of the study protocol and 2 due to OHSS." Number of centres: single‐centre trial Age (y): aged 18‐35: control group: 27.7 ± 1.8, letrozole group: 27.9 ± 1.9 BMI (kg/m²): control group: 25.6 ± 3.2, letrozole group: 25.1 ± 4.4 Duration of infertility (y): control group: 5.4 ± 1.9, letrozole group: 5.0 ± 3.0 Country: Shiraz, Iran
Interventions	Group A: control group. 75 IU/day highly purified recombinant FSH (Gonal‐f, Serono, Hellas, Puregon, Greece) intramuscularly, from the 3rd day through the day of HCG injection Group B: letrozole group additionally received 5 mg/day letrozole (Razak Drug Laboratory, Tehran, Iran) since the 8th day of cycle up to the day of HCG injection.
Outcomes	Premature LH surge, pregnancy rate, abortion rate, ongoing pregnancy rate, number of follicles > 18 mm, endometrial thickness (mm)
Notes	Ethical approval: not reported Informed consent: not reported Source of funding: not reported Power calculation: not reported
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Quote: "In this study, 140 cases with PCOS resistant to CC were enrolled and divided into two groups of control (n = 70) and letrozole (n = 70)."
Allocation concealment (selection bias)	Unclear risk	Not reported
Blinding of participants and personnel (performance bias) All outcomes	Unclear risk	Not reported
Blinding of outcome assessment (detection bias) All outcomes	Unclear risk	Not reported
Incomplete outcome data (attrition bias) All outcomes	Low risk	Quote: "During this study, we eliminated 6 patients from the control group, 4 fell out of the study and 2 were finally diagnosed for OHSS. Three patients were also eliminated from the letrozole group; one fell out of the study protocol and 2 due to OHSS. Thus, 67 patients (age ranges 18‐39 years) remained in control group and 64 cases (age ranges 21‐37 years) remained in letrozole group."
Selective reporting (reporting bias)	Unclear risk	Unclear; quote: "This trial was registered in Islamic Republic Clinical Trials Database (IRCT2014010615102N2)." We were unable to find the protocol, because the given trial registry number leads to a study protocol for pain medication after rhinoplastic surgery.
Other bias	High risk	Methods not very well described, clinical trial registration number leads to wrong trial.