Different types of placebo effects
Dr K K GGARWAL
- 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)