Dear Mark
A good question. The blinding/masking is critical for only *subjective* outcomes, and appears of similar importance to adequate randomization.
So we teach students the3-item "RAMbo" appraisal method:
Randomized?
Attrition (>20% or differential)?
Measurements - blinded OR objective
(the OR is crucial and seems to need repeating so sink in).
The best evidence I know for this is from the 2008 BMJ study (below) which suggested:
"In trials with subjective outcomes effect estimates were exaggerated when there was inadequate or unclear allocation concealment (ratio of odds ratios 0.69 (95% CI 0.59 to 0.82)) or lack of blinding (0.75 (0.61 to 0.93)). In contrast, there was little evidence of bias in trials with objective outcomes: ratios of odds ratios 0.91 (0.80 to 1.03) for inadequate or unclear allocation concealment and 1.01 (0.92 to 1.10) for lack of blinding."
Best wishes
Paul GLasziou
Wood L, Egger M, Gluud LL, Schulz KF, Jüni P, Altman DG, Gluud C, Martin RM, Wood AJ, Sterne JA. Empirical evidence of bias in treatment effect estimates in controlled trials with different interventions and outcomes: meta-epidemiological study. BMJ. 2008 Mar 15;336(7644):601-5. Epub 2008 Mar 3.
<http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2267990/?tool=pubmed>
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From: Evidence based health (EBH) [[log in to unmask]] On Behalf Of Mark V. Johnston [[log in to unmask]]
Sent: 10 February 2011 21:12
To: [log in to unmask]
Subject: How much should study results be downgraded if blinding/masking is incomplete?
We have been debating this question in the Clinical Practice Committee of the American Congress of Rehabilitation.
In past reviews, we had to downgrade studies because of absence of masking or blinding at least as often as for failure to randomize.
Does anyone know of resources on the topic? Do any of the resources provide data to support their opinions?
Explanation:
In the past, we have employed the evidence grading scheme of the American Academy of Neurology (AAN) or the PEDRO system.
- The AAN system downgrades a study if the outcome is not blinded or masked. An unblinded study cannot provide greater than Class 4 or suggestive (level U) evidence, unless the outcome is entirely objective (e.g. mortality, a serum level done by a professional clinical laboratory which follows uniform procedures across patients).
- The PEDRO system is a checklist of study characteristics. It asks whether outcome assessors, clinicians, or patients are blinded, and also total allocation concealment. There are then 4 points for blinding. By comparison, just one point is given for randomization or not.
My opinion: Masking is not 4 times as dangerous as failure to randomize. Blinding is relatively easy in drug studies. (Who knows what's in the pill?) But it is difficult and sometimes impossible to blind therapists and patients so that they do not know what therapeutic activities they are engaged in. Nonetheless, some behavioral or activity therapies are more effective than others, and we need to know which.
We have read many opinions, but we are still looking for resources and data to support our decisions. Hope some of your are interested.
--
Mark V. Johnston, Ph.D.
Professor, Occupational Science and Technology,
College of Health Sciences
University of Wisconsin – Milwaukee
(414) 229-3616
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