Statistics Notes: Blinding In Clinical Trials And Other Studies

Statistics Notes: Blinding In Clinical Trials And Other Studies

BMJ Publishing Group
Statistics Notes: Blinding In Clinical Trials And Other Studies
Author(s): Simon J. Day and Douglas G. Altman
Reviewed work(s):
Source: BMJ: British Medical Journal, Vol. 321, No. 7259 (Aug. 19 - 26, 2000), p. 504
Published by: BMJ Publishing Group
Stable URL: http://www.jstor.org/stable/25225435 .
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Education
and debate
Statistics Notes
in clinical
Blinding
G Alonan
Simon J Day, Douglas
Leo
Human
behaviour
is influenced
Pharmaceuticals,
Princes Risborough,
believe.
In research
there
Buckinghamshire
HP27 9RR
Simon J Day
manager, clinical
biometrics
BMJ 2000;321:504
subjectivity
try to eliminate
It is a tenet
treatment
trial,
to:
we
what
by
assessment,
for
each
irrevocably
bias. This
and
to
biased
is used to
that
trials
is not
patient
been
the
revealed
into
entered
"double blind," usually
in particular
so
keeping
that
study
knowledge.
The
relevance
circumstances.
received
have
tant when
alleviation
such
teria,
not
should
they
of
be
Blinding
patients
in a controlled
trial
to
they
to
trial should be blinded
to minimise
allocation
in assessing
to withdraw
and
management
patient
the decision
For example,
a
in
bias
possible
disease
a
from
study or to adjust the dose of medication could easily
be influenced by knowledge of which treatment group
the
been
has
patient
the patient nor the
In a double blind trial neither
are
caregivers
aware
Blinding
treatment
the
treatment
of
more
means
than
just
Patients
hidden.
to
given
and
being
treatment
the
assignment
the name
keeping
may
the
in
patients
of
see
well
the
other
has
group(s),
In studies
ing
comparing
is possible
using
if we want
example,
as green
presented
could
also
placebo
take one
efficacy.1
two active
blind
compounds,
For
method.
the "double
dummy"
one
two medicines,
to compare
tablets
and
supply green
tablet
green
is
and
one
not
certainly
one
as
capsules,
of patients
groups
pink
pink
we
tablets and pink
placebo
so that both
capsules
Blinding
the
to influence
shown
been
would
sometimes
trials
different
procedures,
surgical
impossible.
ing is often
In a double
assessment
as are
unavoidable,
of
of
styles
or alternative
blind
of patient
easy
or
possible.
trials.
(non-blind)
open
management,
full blind
therapies,
patient
trial it is implicit
outcome
subjec
have
treatments
that
specific
the treatment
occur
their
to
received
staff. Blind
useful
especially
treatment,
when
is
this
to
exposure
subject's
such
studies
blinding
be
discovered
who
participants,
are a case.
disease
patient
events
the
epidemiological
the
may
studies,
such
test
true
of
the
same
the
in
valuable
of
their
of
other
studies.
types of research
the
the
performing
In studies
diagnosis.
a measurement
reproducibility
must
be unaware
observers
to differential
to evaluate
those
exposure
the
study
or not
they
assessment
as cohort
in studies
ance
be
In many
investigated.2 As a
results
in other
is important
each
of important
must
controls
and
Blinded
also
example,
of a diagnostic
of the
be unaware
recall
and if possible
sensitivity
be
considered.
outcome
on
because
cases
between
be
"controls"
interviewing
by
know whether
of
should
Blinding
potential
is
impossible
only
then be recognised
minimum
to
determining
risk factors.
obviously
risk of differential
The
related
as
opposed
are
researchers
while
that the
studies it is preferable
perform
test must
to evaluate
technique
measure
previous
the
individual.
if
the risks of bias
adequate
seem
to be
may
challenging
but bias asso
and patients,
of researchers
subcon
is often
the treatment
with
ciated
knowing
not
that
have
trials
randomised
scious.
On
average,
show
levels
of
used
larger
blinding
appropriate
treatment
than blinded
studies.3
effects
diag
Similarly,
We
have
emphasised
is not used. This
blinding
the integrity
nostic
erence
result.4
test
test
performance
is interpreted
Blinding
or
intentionally
the credibility
of
makes
when
is overestimated
with
it
of
knowledge
to
difficult
unintentionally
study conclusions.
and
so
bias
helps
the ref
the
test
results
ensure
capsule.
always
or
Single blind trials (where either only the investigator
are
to
the
is
blind
the
allocation)
patient
only
In
secret
ments)
of the drug
appearance
a clue to its
Differ
in the study could
used
identity.
give
or mode
also
ences
of delivery
in taste,
smell,
may
so these
be
identical
should
influence
aspects
efficacy,
of medication
colour
treatment
Even
for each
group.
treatment
is
"cases"
too. For
to.
assigned
of
identification
recall
status.
patient
the
the medical
and
of outcome
assessment
can
is particularly
impor
are
such as
criteria
the response
subjective,
for objective
cri
but less important
of
pain,
as death.
for
medical
staff caring
Similarly,
treatment
identify
the patient
kept
vary
according
to the treatment
inevitably
so
are
that
effects
Indeed,
important
involves
some
intentions,
In epidemiological
that
by
the best
of
measure
the outcome
trial.
be more
may
administration
the
blinding
when
especially
the
running
of outcome
be
assessed
a risk.
will
blinding
in a randomised
patients
influenced
in
involved
than
both
those
involved
participants,
and ana
and those
their management,
with
collecting
of the assigned
clinical
data unaware
treatment,
lysing
refers
not
can
lesions
example,
and
after treatment
and
of
are
in trials which
achieved
For
assessment
will
be
assessment
blind
Such
also
before
photographed
someone
by
blind
rence
term
the
trials
often
(non-blinded).
open
tivity. Despite
unintended
better
blinding,
will be discussed
In controlled
received.
can
the
sort of
concealment,
note.
statistics
blinding,
leading
controlled
of randomised
to as allocation
in a future
there is
to
treatment
the
outcome
bias.
such
allocation
referred
or
know
risk of expecta
particular
(sometimes called masking)
has
the patient
to avoid
selection
until
Douglas G Alunan
professor of statistics
inmedicine
Correspondence
SJDay
in
results. Blinding
ICRF Medical
Statistics Group,
Institute of Health
Sciences, Oxford
OX3 7LF
is a
tion influencing findings, most obviously when
some
studies
trials and other
is done
in
that the
ignorance
of
1 De Craen AJM, Roos PJ, de Vries AL, Kleijnen J. Effect of colour of drugs :
systematic review of perceived effect of drugs and their effectiveness. BMJ
1996;313:1624-6.
2 Barry D. Differential recall bias and spurious associations in case/control
studies. StatMed 1996;15:2603-16.
3 Schulz KF, Chalmers I,Hayes R, Alunan DG. Empirical evidence of bias:
dimensions of methodological quality associated with estimates of treat
ment effects in controlled trials./AM4 1995;273:408-12.
4 Lijmer JG, Mol BW, Heisterkamp S, Bonsel GJ, Prins MH, van der
MeulenJH, et al. Empirical evidence of design-related bias in studies of
diagnostic tests.JAMA 1999;282:1061-6.
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