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The Double-Blind Design

How do we deal with experimenter effects? We want to be able to test new treatments and techniques, but we do not want to be misled by results that are due to an experimenter's belief, knowledge, or enthusiasm.

What is a double-blind design?

The solution is to arrange an experimental set-up in which the experimenter does not know which subjects are receiving which treatment. This is a double-blind design. The subjects do not know which group they are in, and neither does the experimenter or the person collecting data. So both subjects and researchers are kept blind about which group a subject is in, until the data-collection phase of the research is over.

Why should double-blind methodology perhaps be called "triple blind"?

Perhaps double-blind experimental methodology should be called triple-blind. Experimenter effects must be eliminated during both the treatment and data collection phases. These operations (treatment and data collection) are not necessarily performed at the same time or by the same people. In a double-blind experiment neither (1) the subject, nor (2) the person giving the treatment, nor (3) the person collecting data knows which treatment a subject received...until after the data are collected. "Blind" methodology is appropriate whenever awareness of the subject's experimental group membership could alter the data .

How did psychologists use double-blind methodology to test the effectiveness of subliminal learning tapes?

An example of double-blind research is a study of subliminal learning tapes (Greenwald, Spangenberg, Pratkanis, and Eskenazi, 1991). These are tapes that supposedly program the unconscious mind by giving very weak instructions, too quiet to be heard, while a person is resting or sleeping. The researchers gave tapes to subjects who used them for a month. The tape label indicated the tape was for (1) self-esteem improvement, or (2) memory improvement.. Half the tapes were deliberately mislabeled. Neither the researchers nor the subjects knew which tapes had the wrong label until after data was collected.

Many of the subjects claimed that the tapes helped them in self-esteem or memory improvement. However, these "improvements" corresponded to the label, not to the actual contents of the tape. A student who received a self-esteem improvement tape, mislabeled as a memory improvement tape, would typically report improvements in memory but not self-esteem. In this way the researchers demonstrated subliminal self-help tapes had no real effect beyond the placebo effect. If the researchers had not used a double-blind procedure, they might have concluded that the tapes were effective.

In general, when should double-blind methods be used?

Experimenter effects are relevant in every realm of science, and double-blind methodology should be used in any form of research where an experimenter expects or favors a particular outcome from an experiment. For example, the editors of Stereo Review found that expert listeners typically rated the sound of a high-priced CD player as better than that of a lower-priced CD player...but only if they knew which equipment was producing the sound! When double blind methods were employed, the expert listeners found that all the CD players sounded the same (Master, 1986).

What often happens, after an exciting pilot study? Why is this a good thing?

Earlier we referred to the tendency of false claims to produce diminishing returns as replications are conducted with better and better controls. Eventually the effect cannot be reproduced at all...because it never existed in the first place. Rather, it was due to some error or some form of biased observation. Similarly, the results of "pilot studies" or informal clinical observations may lead to great excitement, but when proper controls such as double-blind procedures are put into place, the exciting finding may disappear.

How did researchers find out that hemodialysis did not really help schizophrenics?

This is exactly what happened with a study that explored the effects of hemodialysis (the blood-cleansing procedure) on schizophrenia (a serious mental illness). Initial studies indicated promising effects, but when double-blind methods were employed in a full-scale clinical trial, there was no effect at all. Such failures to replicate early findings can be discouraging. But such "failures" are the reason science marches forward. Theories are not accepted just because a treatment works in the field or in a clinical setting. Scientists want to know why something works, so after a promising field trial, they put controls into place to eliminate placebo effects and experimenter effects, and sometimes the effect disappears. If an effect disappears when controls are tightened, we are better off to know this, rather than to believe in a treatment that does not really work.


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