Critical appraisal:Cook NR, Lee IM, Zhang SM, Moorthy MV, Buring JE 2013 1

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Risk of bias assessment: Randomised Controlled Trial (Cochrane risk of bias tool)

Random sequence generation
Describe the method used to generate the allocation sequence in sufficient detail to allow an assessment of whether it should produce comparable groups.Jutta's question mark icon.png
The 65 169 women who were willing and eligible entered a 3-month run-in period using both placebo aspirin and placebo vitamin E to identify likely long-term compliers to pill taking. A total of 39 876 women remained willing and eligible, were compliant during run-in, and were randomized into the trial (19 934 to aspirin and 19 942 to placebo). Randomization used blocks of size 16 within 5-year age strata and took place from April 30, 1993, through January 24, 1996.
What was the risk of bias from the random sequence generation?Jutta's question mark icon.png
Allocation concealment
Describe the method used to conceal the allocation sequence in sufficient detail to determine whether intervention allocations could have been foreseen in advance of or during, enrolment.Jutta's question mark icon.png
Participants were sent an annual supply of monthly calendar packs containing active agents or placebo. Every 6 months for the first year, then every 12 months subsequently, participants were sent questionnaires seeking information on compliance, adverse effects, occurrence of relevant clinical end points, and risk factors. Study medications and end point ascertainment were continued in blinded fashion through the scheduled end of the trial on March 31, 2004.
What was the risk of bias from the allocation concealment?Jutta's question mark icon.png
Describe all measures used, if any, to blind outcome assessors from knowledge of which intervention a participant received. Provide any information relating to whether the intended blinding was effective.Jutta's question mark icon.png
All participants were followed up for the occurrence of cancer or cardiovascular events. Following a report of cancer by questionnaire or death certificate, written consent for medical record review was requested from the participant, or next of kin if deceased, and medical records were obtained from hospitals or treating physicians. All relevant information was reviewed by the WHS Endpoints Committee composed of physicians blinded to treatment assignment.
What was the risk of bias from the blinding of participants and personnel and outcome assessors?Jutta's question mark icon.png
Incomplete outcome data
Describe the completeness of outcome data for each main outcome, including attrition and exlusions from the analysis. State whether attrition and exclusions were reported, the numbers in each intervention group (compared with total randomized participants), reasons for attrition/exclusions where reported, and any re-inclusions in analyses performed by the review authors.Jutta's question mark icon.png
Attrition due to death was just under 5% for both groups. Participated that opted out from further inclusion was only 11%. Large data set start with just under 40,000. Among those opting in for continued foloow-up, no based differences between groups.
What was the risk of bias from incomplete outcome data?Jutta's question mark icon.png
Selective outcome reporting
State how the possibility of selective outcome reporting was examined by the review authors and what was found.Jutta's question mark icon.png
Outcomes clearly stated for the trial. Some data based on 'self-reported' diagnosis, but this was confirmed by looking at medical records. Low risk of bias.
What was the risk of bias from selective outcome reporting? Assessments should be made for each main outcome (or class of outcomes).Jutta's question mark icon.png
Other sources of bias
Describe any other sources of biasJutta's question mark icon.png
Some data set are not complete. But its a very large data set followed for a long time (18 years). The authors used 'sensitivity analysis' to compensate for imbalance from the opt-out patients.
What was the risk of bias from other sources?Jutta's question mark icon.png
Overall risk of bias
Low risk of bias Additional comments: Please replace this text and include any additional comments in regards to your risk of bias rating

Jutta's tick icon.png This appraisal has been completed.

Cook NR, Lee IM, Zhang SM, Moorthy MV, Buring JE. Alternate-day, low-dose aspirin and cancer risk: long-term observational follow-up of a randomized trial. Ann Intern Med 2013 Jul 16;159(2):77-85 Available from:
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Topic area
Guidelines:Colorectal cancer/Primary Prevention
Clinical question
Form:Quality appraisal rct-cochrane
CRC Incidence

Section below only relevant for Cancer Council Project Officer

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