Table 1.

Methods to reduce the risk of selection bias in non-double-blind clinical trials*

Techniques	Rationale	Effect on risk of selection bias
Use blinded recruiters	If recruiters are blind to previous trial allocations, they will be unable to predict the next allocation	Risk of selection bias will be eliminated provided the blinding is maintained
Use simple (unrestricted) randomisation	Recruiters cannot guess the next allocation with any degree of accuracy	Risk of selection bias will be eliminated
If restricted randomisation is used, do not stratify by site of recruitment	The probability of the allocation will depend on previous allocations at other sites, which recruiters are unlikely to have access to, making an accurate guess more difficult	Risk of selection bias will be reduced, but not necessarily eliminated
When randomisation is stratified by site, avoid permuted blocks	Permuted blocks stratified by site will maximise the probability of correctly guessing the next allocation. Using alternative randomisation methods will reduce the probability of correctly guessing the next allocation	Risk of selection bias will be reduced, but not necessarily eliminated
When restricted randomisation is used, stratify by prognostic covariates as well	There is typically less variation in prognoses for patients with the same covariate pattern, making it more difficult for investigators to identify patients with a specific prognosis to enrol into the trial when their preferred treatment is more likely	Risk of selection bias will be reduced, but not necessarily eliminated

*This table assumes that allocation concealment has been appropriately implemented