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Devereaux PJ, Manns BJ, Ghali WA, et al. Physician Interpretations and Textbook Definitions of Blinding Terminology in Randomized Controlled Trials. JAMA. 2001;285(15):2000–2003. doi:10.1001/jama.285.15.2000
Context When clinicians assess the validity of randomized controlled trials
(RCTs), they commonly evaluate the blinding status of individuals in the RCT.
The terminology authors often use to convey blinding status (single, double,
and triple blinding) may be open to various interpretations.
Objective To determine physician interpretations and textbook definitions of RCT
Design and Setting Observational study undertaken at 3 Canadian university tertiary care
centers between February and May 1999.
Participants Ninety-one internal medicine physicians who responded to a survey.
Main Outcome Measures Respondents identified which of the following groups they thought were
blinded in single-, double-, and triple-blinded RCTs: participants, health
care providers, data collectors, judicial assessors of outcomes, data analysts,
and personnel who write the article. Definitions from 25 systematically identified
textbooks published since 1990 providing definitions for single, double, or
Results Physician respondents identified 10, 17, and 15 unique interpretations
of single, double, and triple blinding, respectively, and textbooks provided
5, 9, and 7 different definitions of each. The frequencies of the most common
physician interpretation and textbook definition were 75% (95% confidence
interval [CI], 65%-83%) and 74% (95% CI, 52%-90%) for single blinding, 38%
(95% CI, 28%-49%) and 43% (95% CI, 24%-63%) for double blinding, and 18% (95%
CI, 10%-28%) and 14% (95% CI, 0%-58%) for triple blinding, respectively.
Conclusions Our study suggests that both physicians and textbooks vary greatly in
their interpretations and definitions of single, double, and triple blinding.
Explicit statements about the blinding status of specific groups involved
in RCTs should replace the current ambiguous terminology.
To assess the validity of a randomized controlled trial (RCT), clinicians
should note whether participants, health care providers, data collectors,
and those assessing occurrence of target events of interest (judicial assessors
of outcomes) are blind to participant allocation to treatment or control.1,2 Some methodologists have suggested
that allocation should also be concealed from data analysts and personnel
writing the paper.3,4 Authors
of RCTs frequently use the terms "single," "double," and "triple" blind to
communicate the blinding status of persons involved in the trials. We suspected
that physicians and textbooks vary in their interpretations and definitions
of these terms. Therefore, we surveyed physicians and systematically reviewed
textbooks to test this hypothesis. A review of 200 RCTs provided an estimate
of the clarity with which investigators identify who is blinded in studies
they describe as single-, double-, and triple-blind.
We surveyed attending physicians between February and May 1999 within
departments of medicine at 3 Canadian university tertiary care centers: Dalhousie
University, McMaster University, and University of Calgary Foothills Hospital.
Respondents completed a survey that defined the 6 groups who are potential
candidates for blinding in an RCT: participants, health care providers, data
collectors, data analysts, judicial assessors of the outcomes (those ultimately
deciding whether a participant meets the criteria for a study's outcome),
and personnel writing the paper (those writing either of 2 drafts of a paper
prior to the breaking of the randomization code, with draft 1 written assuming
that group A is the treatment group and draft 2 assuming that group B is the
treatment group). We randomized the order in which the 6 groups were listed.
Respondents offered their opinions concerning which group(s) is blinded in
single-, double-, and triple-blinded trials. Physicians could include as many
groups as they thought appropriate for each blinding term but they were only
allowed to provide 1 definition per term.
Using the terms clinical epidemiology, randomized controlled trial, and evidence-based
medicine, we systematically identified textbooks published since 1990
with definitions for single, double, and/or triple blinding at the university
libraries of McMaster and Dalhousie Universities. Two of us (P.J.D., C.L.)
independently evaluated the textbooks, recorded all definitions of the blinding
terminology, and resolved disagreements by consensus. The chance-correct agreements,
assessed through means of a κ statistic, were near perfect (κ≥0.9).
Starting with June 2000, we systematically identified the 40 most recent
RCTs published in the Annals of Internal Medicine, BMJ, JAMA, The Lancet, and The New England Journal of Medicine. Two of us (V.M.M.,
M.B.) independently evaluated the RCTs for reporting of single, double, and
triple blinding and the blinding status of the 6 groups identified above.
Duplicate evaluation of the first 60 RCTs established near-perfect agreement
for all variables assessed (κ≥0.8).
We determined the proportion of physician respondents who chose each
interpretation, the frequency of the various textbook definitions, and the
associated 95% confidence intervals (CIs). We also performed the Fisher exact
test to assess heterogeneity in physician interpretation of single, double,
and triple blinding across the 3 sites.5 To
avoid cells with sparse data, we collapsed all but the 2 most frequently chosen
options into a single category when conducting these tests. We determined
the proportion of RCTs reporting the terms single, double, and triple blinding;
within these studies we then determined the frequency of blinding among the
groups identified above.
All 23 Dalhousie University cardiologists, 46 of the 52 McMaster University
general internists, and 22 of the 24 University of Calgary Foothills Hospital
general internists completed the survey (response rate, 92%). Of the 91 respondents,
83 provided an interpretation of triple blinding.
Of the 25 textbooks2,3,6-28
that fulfilled our entry criteria, 17, 25, and 5 provided a definition for
single, double, and triple blinding, respectively. A number of textbooks provided
2 definitions for single (n = 6), double (n = 3), and triple (n = 2) blinding.
Table 1 demonstrates that
75% (95% CI, 65%-83%) of physicians believed and 74% (95% CI, 52%-90%) of
textbooks reported that study subjects were unaware of allocation in single-blind
studies. The remaining 25% of physicians chose 9 alternative interpretations;
26% of textbooks provided 4 alternative definitions. Physicians chose 17 different
interpretations of double blinding and textbooks provided 9 different definitions,
the most frequent of which, blinding of both patients and their caregivers,
was chosen and defined by 38% (95% CI, 28%-49%) and 43% (95% CI, 24%-63%)
of physicians and textbooks, respectively. Physicians identified 15 different
interpretations and textbooks provided 7 unique definitions for triple blinding.
Only 18% (95% CI, 10%-28%) of the physicians and 14% (95% CI, 0%-58%) of the
textbooks selected or defined the most frequent category of triple blinding.
The analyses for heterogeneity in physician interpretation of blinding
terminology demonstrated similar variation in interpretation across sites,
with P = .96 for single blinding, P = .91 for double blinding, and P = .13 for
Of the 200 RCTs evaluated, 5 reported being single-blind and 83 double-blind.
In the 5 studies reported as single-blind, 1 study made no mention of which
group was blinded, 2 studies identified 1 group as blinded, 1 study identified
2 groups as blinded, and 1 study identified 3 groups as blinded. In the 83
studies reported as double-blind, 41 made no mention of which groups were
blinded, 29 studies identified 1 group as blinded, 11 studies identified 2
groups as blinded, 1 study identified 3 groups as blinded, and 1 study identified
4 groups as blinded.
While methodologists advocate assessing the blinding status of individuals
involved in an RCT when determining trial validity,1-3,13,16,19,24,29
clinicians depend on authors and editors to present blinding information in
a transparent form. Authors often rely on the terms "single," "double," and
"triple" blind to convey the blinding status of individuals involved in their
study. Indeed, these terms, particularly double blind, have become almost
a matter of convention.30
Our study demonstrates important variation among physicians in their
interpretation of who is unaware of allocation when authors use the terms
"single," "double," and "triple" blind. This variability suggests that the
current terminology hinders readers who are trying to accurately assess trial
validity. The variation in textbook definitions demonstrates that there is
no consensus among authorities as to what these terms mean. Our survey of
200 recent RCTs in high-impact journals demonstrated that, in 50% of trials,
authors who use the term "double-blind" failed to make any statement about
who was blinded, and specified only 1 group—likely omitting information
about 1 or more other groups who were blinded—in another 35%.
Given the variability we found, the current blinding terminology is
failing to achieve the basic objectives of clear research communication. The
likelihood that knowledge of patient allocation introduces bias makes this
problem serious. The placebo effect can inflate the size of the treatment
effect in studies when participants are not blinded.31,32
When unblinded, clinicians may differentially administer powerful treatments
other than those under study, influence a patient's compliance with study
medication or willingness to continue in the study, and affect patient reporting
of symptoms.1,33 Unblinded data
collectors can distort trial results based on intensity of examination, the
likelihood of repeating a test for an unexpected finding, the recording of
outcomes, or differential encouragement during performance testing.2,3,34 Unblinded judicial
assessors can bias the interpretation of marginal findings.1,2
Unblinded data analysts and authors can introduce bias through decisions on
patient withdrawals, selection of outcomes to analyze or report, choice of
time points demonstrating maximum or minimum effects, and a myriad of other
decisions in the analysis or reporting process.3,4,35,36
The impact of the blinding status of each of these groups remains uncertain.
The only studies assessing the influence of blinding status on trial outcomes
have focused on the reporting of double blinding. This research has demonstrated
conflicting findings: 2 studies have suggested that RCTs without mention of
double blinding are more likely to favor the experimental group than RCTs
with mention of double blinding,37,38
whereas 1 study failed to confirm this association.30
Variability in who was actually blinded in reports of double-blind trials
may account for these discrepant findings.
Our study is limited in that we surveyed physicians within the departments
of medicine at only 3 academic institutions in Canada. However, given that
McMaster University has made special efforts to educate its internists in
critical appraisal of the medical literature, one would expect the highest
likelihood of consistent interpretation in this institution. The results are
remarkably similar across the 3 sites of our survey and are consistent with
the varied definitions provided by textbooks, suggesting that our findings
may be widely generalizable.
Our study has demonstrated enormous ambiguity in the conventional ways
of describing blinding. Our results suggest that authors and journal editors
should abandon the terms single, double, and triple blind, and substitute
descriptions stating which of the relevant groups were unaware of allocation.
This change in reporting would be consistent with the Consolidated Standards
of Reporting Trials (CONSORT) statement, which calls for reporting of the
blinding status of the specific groups involved in RCTs.39
As long as journal reports of RCTs include the term "double blind," clinicians
will risk inaccurate inferences about the validity of the studies on which
they base their clinical practice.
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