GISP 1997.

Methods	Randomised controlled trial.
Participants	112 participants with polycythaemia vera (42 females, 70 males; aged 17 to 80 years), in whom physicians were uncertain whether aspirin was indicated. Participants were ineligible if physicians were reasonably certain that aspirin was indicated, was not indicated, or was contraindicated. Moreover, exclusion criteria included stage III chronic renal failure, active gastrointestinal disease, and a history of a major haemorrhagic episodes during the preceding four months.
Interventions	Low‐dose aspirin, 40 mg per day (N = 60), versus placebo (N = 52) (in addition to the standard management policies adopted by the participating centres).
Outcomes	Primary endpoints: major haemorrhagic events, compliance with the assigned treatment, gastric intolerance. Secondary endpoints: acute myocardial infarction, transient ischaemic attacks, stroke, venous thromboembolism, and acute arterial thrombosis. Follow‐up duration: 16 ± 6 months.
Notes	No data on concomitant cytoreductive therapy are provided in the manuscript.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	'... Participants were randomly allocated ... Randomisation was double blind, centrally co‐ordinated, and stratified by centre and according to thrombotic risk ...'
Allocation concealment (selection bias)	Low risk	'... Randomisation was double blind, centrally co‐ordinated, and stratified by centre and according to thrombotic risk ...'
Incomplete outcome data (attrition bias) All outcomes	Unclear risk	This judgement was based on the fact that selective loss to follow‐up or study withdrawal (8% of participants) could not be excluded because data in the published article were insufficient. The authors provided no further data.
Blinding of participants and personnel (performance bias) All outcomes	Low risk	'... Participants were randomly allocated to receive active treatment, ..., or placebo ... Randomisation was double blind, centrally co‐ordinated, and stratified by centre and according to thrombotic risk ...'
Blinding of outcome assessment (detection bias) All outcomes	Low risk	'... Randomisation was double blind, centrally co‐ordinated, and stratified by centre and according to thrombotic risk ... All randomised participants were seen by their haematologists every 3 months and underwent a clinical and laboratory evaluation of their status ... '