[Наталья П.]

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Uncontrolled studies


To measure the effect of a food supplement on headaches, a questionnaire survey was sent to a random sample of workers in one factory. People who scored in the top 20% for frequency of headaches in the previous two weeks were invited to "try a natural food supplement used to treat headaches." A follow-up questionnaire two weeks later showed markedly fewer headaches. Which, if any, of the following could affect the results?


a) The Hawthorne effect

b) Regression to the mean

c) Healthy worker effect

d) Placebo effect

e) Confounding by indication