It can be quite confusing!
You are right in most cases that a non-inferiority trial is likely to
demonstrate no clinically meaningful differences between the interventions
compared (if it meets its primary outcome).
However, a non-inferiority design does not preclude the possibility of one
intervention being superior to another. In a sense a non-inferiority study
is an "over powered" superiority study.
One thing to be careful of is that non-inferiority is based on per protocol
results (to prevent attrition bias) while superiority should be based on an
I hope that helps.
Dr Steven J. Edwards DPhil MSc BSc (Hons)
Head of Health Technology Assessment
BMJ Technology Assessment Group (BMJ-TAG)
BMJ Evidence Centre
BMJ Group, BMA House
London WC1H 9JR
Tel: +44 (0) 207 383 6112
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From: Marsh Roy <[log in to unmask]>
To: [log in to unmask]
Date: 18/01/2012 16:05
Subject: Query re non-inferiority studies
Sent by: "Evidence based health (EBH)"
<[log in to unmask]>
This question was posed to me by a public health information analyst the
"Can you work out NNT in a non-inferiority trial?'
I think not. Because the finding is equivalence. So you can't do it (can
But I have come across trials that call themselves 'non-inferiority' but
clearly show superiority, at least for some outcomes. And work out NNT for
those outcomes (though not for the equivalent outcomes).
Presumably the trials had to demonstrate that they were powered enough to
detect superiority ... and then went and found it (even though that wasn't
the actual aim)?
Why call themselves 'non-inferiority' anyway, if they have to be as
powerful as 'normal' trials? What does 'non-inferiority' add to my
understanding of a trial?
example: Diggle L, Deeks JJ, Pollard AJ. Effect of needle size on
immunogenicity and reactogenicity of vaccines in infants: randomised
controlled trial. BMJ 2006;333:571.
I bow before greater minds. Especially if the answer is simple.
Roy Marsh, Research Fellow
Evidence Adoption Centre
Douglas House, 18 Trumpington Road, Cambridge, CB2 8AH
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