BASIL study.

Methods	Study design: randomised, no statement of blinding Method of randomisation: by computer, random numbers, sealed envelope Power calculation performed: 90% power, alpha = 0.05 Losses to follow up: 4 Intention‐to‐treat analysis: yes
Participants	Country: UK Setting: 27 centres Number of participants: 452 Age: median age 75 years (interquartile range 67 to 82) Sex: male and female Inclusion criteria: severe limb ischaemia defined as rest pain or tissue loss of presumed arterial aetiology for more than 2 weeks and treatable by bypass or balloon angioplasty Exclusion criteria: supra‐inguinal disease, pre‐existing medical condition that makes revascularization inappropriate
Interventions	Treatment: infra‐inguinal bypass surgery Control: balloon angioplasty Duration: all participants were monitored for 3 years and more than half for > 5 years
Outcomes	Primary outcomes: time to amputation of trial leg or death (whichever occurred first) Secondary outcomes: all‐cause mortality, 30‐day morbidity and mortality, reinterventions, health‐related quality of life, use of hospital resources
Notes	This trial was funded by the UK NHS Research and Development Health Technology Assessment programme.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Quote: “The randomisation sequences were generated by a computerised random‐number generator in the University of Edinburgh Medical Statistics Unit (Edinburgh, UK) and supplied to the coordinating centre in identical, sealed envelopes”. Comment: Appropriate method of randomisation.
Allocation concealment (selection bias)	Low risk	Central allocation, as described above.
Blinding (performance bias and detection bias) All outcomes	Low risk	See below.
Blinding of participants and personnel (performance bias) All outcomes	Low risk	No blinding, but outcomes unlikely to be influenced by lack of blinding.
Blinding of outcome assessment (detection bias) All outcomes	Low risk	No mention that the personnel measuring the outcome were blinded. However, the primary outcome measures were objective and unlikely to be affected. The final data were checked using NHS sources, hospital and GP records. Endpoint data for death and amputation were also collected via the national audit mechanism. Postoperative complications and reinterventions data were collected by 4 dedicated research nurses during the first year, travelling to all centres; thereafter, yearly by the trial coordinator visiting each centre.
Incomplete outcome data (attrition bias) All outcomes	Low risk	Apart from 4 participants lost to follow up, there was a three‐year complete follow‐up for all participants.
Selective reporting (reporting bias)	Low risk	The study predefined primary and secondary outcomes were reported.
Other bias	Low risk	No other source of bias was identified.