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Science Forum Index » Philosophy Forum » Blind experiment
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| turtoni |
 Posted: Sat May 03, 2008 6:24 pm |
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"The blind method is a part of the scientific method, used to prevent
research outcomes from being influenced by either the placebo effect
or the observer bias. Blinded research is an important tool in many
fields of research, from medicine, to psychology and the social
sciences, to forensics.
Single-blind trials
Blinding is a basic tool to prevent conscious and subconscious bias in
research. For example, in open taste tests comparing different product
brands, consumers usually choose their regular brand. However, in
blind taste tests, where the brand identities are concealed, consumers
may favor a different brand.
Single blind describes experiments wherein information that could
introduce bias or otherwise skew the result is withheld from the
participants. Unlike in a double blind experiment, the experimenter
will be in full possession of the facts. A trivial example of a single-
blind experiment would be the Pepsi Challenge.
In a single blind experiment, the individual subjects do not know
whether they are so-called "test" subjects or members of an
"experimental control" group. Single-blind experimental design is used
where the experimenters either must know the full facts (for example,
when comparing sham to real surgery) and so the experimenters cannot
themselves be blind, or where the experimenters will not introduce
further bias and so the experimenters need not be blind. However,
there is a risk that subjects are influenced by interaction with the
researchers — known as the experimenter's bias. Single-blind trials
are especially risky in psychology and social science research, where
the experimenter has an expectation of what the outcome should be, and
may consciously or subconsciously influence the behavior of the
subject.
Double-blind trials
Double-blind describes an especially stringent way of conducting an
experiment, usually on human subjects, in an attempt to eliminate
subjective bias on the part of both experimental subjects and the
experimenters. In most cases, double-blind experiments are held to
achieve a higher standard of scientific rigour.
In a double-blind experiment, neither the individuals nor the
researchers know who belongs to the control group and the experimental
group. Only after all the data has been recorded (and in some cases,
analyzed) do the researchers learn which individuals are which.
Performing an experiment in double-blind fashion is a way to lessen
the influence of the prejudices and unintentional physical cues on the
results (the placebo effect, observer bias, and experimenter's bias).
Random assignment of the subject to the experimental or control group
is a critical part of double-blind research design. The key that
identifies the subjects and which group they belonged to is kept by a
third party and not given to the researchers until the study is over.
Double-blind methods can be applied to any experimental situation
where there is the possibility that the results will be affected by
conscious or unconscious bias on the part of the experimenter.
Computer-controlled experiments are sometimes also referred to as
double-blind experiments, since software should not cause any bias. In
analogy to the above, the part of the software that provides
interaction with the human is the blinded researcher, while the part
of the software that defines the key is the third party. An example is
the ABX test, where the human subject has to identify an unknown
stimulus X as being either A or B.
"Triple-blind" trials
Triple-blind trials are double-blind trials in which the statistician
interpreting the results also does not know which intervention has
been given. Sometimes triple-blind is used to mean that multiple
investigators are all blinded to the protocol (such as the clinician
giving the treatment and a radiologist or pathologist who interprets
the results.) The use of the term triple-blind experiments is
disputed.
Medical applications
Double-blinding is relatively easy to achieve in drug studies, by
formulating the investigational drug and the control (either a placebo
or an established drug) to have identical appearance (color, taste,
etc.). Patients are randomly assigned to the control or experimental
group and given random numbers by a study coordinator, who also
encodes the drugs with matching random numbers. Neither the patients
nor the researchers monitoring the outcome know which patient is
receiving which treatment, until the study is over and the random code
is broken.
Effective blinding can be difficult to achieve where the treatment is
notably effective (indeed, studies have been suspended in cases where
the tested drug combinations were so effective that it was deemed
unethical to continue withholding the findings from the control group,
and the general population),[citation needed] or where the treatment
is very distinctive in taste or has unusual side-effects that allow
the researcher and/or the subject to guess which group they were
assigned to. It is also difficult to use the double blind method to
compare surgical and non-surgical interventions (although sham
surgery, involving a simple incision, might be ethically permitted). A
good clinical protocol will foresee these potential problems to ensure
blinding is as effective as possible.
Evidence-based medicine practitioners prefer blinded randomised
controlled trials (RCTs), where that is a possible experimental
design. These are high on the hierarchy of evidence; only a meta
analysis of several well designed RCTs is considered more reliable.
Nuclear and particle physics
Modern nuclear physics and particle physics experiments often involve
large numbers of data-analysts working together to extract
quantitative data from complex datasets. In particular, the analysts
want to report accurate systematic error estimates for all of their
measurements; this is difficult or impossible if one of the errors is
observer bias. To remove this bias, the experimenters devise blind
analysis techniques, where the experimental result is hidden from the
analysts until they've agreed—based on properties of the data set
other than the final value—that the analysis techniques are fixed.
One example of a blind analysis occurs in neutrino experiments, like
the Sudbury Neutrino Observatory, where the experimenters wish to
report the total number N of neutrinos seen. The experimenters have
preexisting expectations about what this number should be, and these
expectations must not be allowed to bias the analysis. Therefore, the
experimenters are allowed to see an unknown fraction f of the dataset.
They use this data to understand the backgrounds, signal-detection
efficiencies, detector resolutions, etc.. However, since no one knows
the "blinding fraction" f, no one has preexisting expectations about
the meaningless neutrino count N' = N x f in the visible data;
therefore, the analysis does not introduce any bias into the final
number N which is reported. Another blinding scheme is used in B meson
analyses in experiments like BaBar and CDF; here, the crucial
experimental parameter is a correlation between certain particle
energies and decay times—which require an extremely complex and
painstaking analysis—and particle charge signs, which are fairly
trivial to measure. Analysts are allowed to work with all of the
energy and decay data, but are forbidden from seeing the sign of the
charge, and thus are unable to see the correlation (if any). At the
end of the experiment, the correct charge signs are revealed; the
analysis software is run once (with no subjective human intervention),
and the resulting numbers are published. Searches for rare events,
like electron neutrinos in MiniBooNE or proton decay in Super-
Kamiokande, require a different class of blinding schemes.
The "hidden" part of the experiment—the fraction f for SNO, the charge-
sign database for CDF—is usually called the "blindness box". At the
end of the analysis period, one is allowed to "unblind the data" and
"open the box".
Forensic application
In a police photo lineup, an officer shows a group of photos to a
witness or crime victim and asks him to pick out the suspect. This is
basically a single-blind test of the witness' memory, and may be
subject to subtle or overt influence by the officer. There is a
growing movement in law enforcement to move to a double blind
procedure in which the officer who shows the photos to the witness
does not know which photo is of the suspect."
Brian i suspect you were overly "exposed" to the supplement culture in
body building and you now have an aversion to any notion of a
supplement.
Which supplements are you cool with? I take it you wouldn't take any
form of medication?
Please make sure you supply an obscure mystical non-answer as if you
were running for political office. |
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