Different types of placebo effects

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· The placebo effect in medicine, where getting an inert (e.g. sugar) pill has a large positive effect. It is believed that often there are large positive effects that are simply from the expectation created in the patient. If true, this is the placebo effect, where the intervention in question has no material effect, but the belief of the patient does. Although often transmitted from the doctor’s expectancies, it may be independent of the doctor. The placebo effect may be particularly strongly evident in side-effects, where the number and severity of side-effects may be three times larger when patients are warned about the possibility in the study group and in the placebo group.

· The Hawthorne effect (French, 1953) or the observer effect i.e. the effect did not depend on the particular expectation of the researchers, but being studied caused the improved performance. This might be because attention made the workers feel better; or because it caused them to reflect on their work and reflection caused performance improvements, or because the experimental situation provided them with performance feedback they didn’t otherwise have and this extra information allowed improvements.

· The John Henry effect (Zdep & Irvine, 1970; Saretsky, 1972) is the opposite of the Hawthorne effect. It is seen when a supposedly control group, that receives no intervention compares itself to the experimental group and through extra effort has similar effects or results. It is a type of counter-suggestibility.

· Jastrow’s effect: Here an explicit expectation about performance is transmitted and turned out to change output by a factor of three. (Rosenthal & Jacobson, 1968; Jastrow, 1900.)

· The Pygmalion effect or “expectancy advantage” is that of a self-fulfilling prophecy. Teachers’ expectations of pupils can strongly affect (by about a factor of two over a year) the amount of development they show. (Rosenthal & Jacobson, 1968)

· The Charisma effect. This term is most used as one of the rival theories of leadership.

· The Halo effect. Coined by Thorndike, this is a specific type of cognitive confirmation bias, wherein positive feelings in one area cause ambiguous or neutral traits to be viewed positively. In other words, as in the use of brands in marketing, positive attitudes can spread from one aspect or thing to associated ones. E.g. from the researcher to the task; from the use of new technology to the amount you are learning.

· The Novelty effect: The participant performs differently at first because of the novelty of the “treatment” which may change their expectation, or simply cause them to be more alert or otherwise perform differently. The experimenter is not important, but a materially unjustified belief, perhaps from other social media may be e.g. participants think the technology / educational intervention is wonderful and that belief is the real cause of raised outcomes or else simply the novelty rather than belief matters, if it operates through attention rather than through expectancies.

· Experimenter effects. Specific expectations acquired, consciously or not, from the researcher. Some experimenter effects have been demonstrated equally in positive and negative directions.

· Trial effects. Similar to experimenter effects, but with the emphasis on the activity and not on the experimenter.

· Research participation effects. Similar, but perhaps a more general label for observer, experimenter, trial, and novelty effects

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