Access to price information shifts the balance of power to the buyers: they can buy as little or as much as they need, treating the order form as if it were a spreadsheet, to see the effect of volume discounts or of adding or removing features.

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One conclusion emerging from the research, says Irving Kirsch, a professor at Harvard Medical School who wrote the preface to the volume, is that the effect is strongest for those disorders that are predominantly mental and subjective, a conclusion backed by a meta-analysis of placebo studies that was carried out in 2010 by researchers at the Cochrane Collaboration, an organisation that reviews evidence for medical treatments.

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Researchers hope the sheer volume of participants will wash out the effect of bad data.

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