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The answer: 40 out of 50, a list that includes salt, flour, parsley and sugar. "All we eat is linked to cancer," they quirk in their 2013 article.
Your question is related to a known but persistent problem in the research world: many studies use Samples are too small to reach generalizable conclusions.
But the pressure on researchers, the competition between magazines and the insatiable media appetite for studies that announce revolutions or great discoveries make these articles continue to be published.
"Most of the articles published, even in serious journals, are lax," says one of the authors, John Ioannidis, a professor of medicine at Stanford, who specializes in studying studies.
This detractor of bad scientific research demonstrated in a 2005 article "why most published studies are wrong".
Since then, he says, only some progress has been made.
Some journals require that authors provide their raw data and publish their protocol in advance. This transparency prevents researchers from twisting their methods and data to find a result, no matter what.
It also allows others to check or "replicate" the study.
Because when they are redone, the experiments rarely lead to the same results.
Only one third of the 100 studies published in the three most prestigious psychology journals were reproduced by researchers, in an analysis published in 2015.
Medicine, Epidemiology, Drug Clinical Trials and Nutrition Studies does not work much better, Ioannidis insists, especially during rehearsals.
"In the biomedical sciences and elsewhere, scientists do not train enough in statistics and methodology," he adds.
Too many studies focus on a few individuals, making it impossible to generalize to a total population, since selected participants are unlikely to be representative.
Coffee and Red Wine [19659002] "The diet is one of the most regrettable areas," continues Professor Ioannidis, and not just because of conflicts of interest with the # 39, agro-food industry.
Researchers often look for correlations in huge databases, without a basic assumption.
In addition, "measuring a diet is extremely difficult," he explains. How to quantify exactly what people eat?
Even when the method is good, with a randomized study where participants are chosen at random, the execution sometimes leaves something to be desired.
A famous 2013 study on the benefits of the Mediterranean diet on heart disease had to be withdrawn in June by the prestigious medical journal The New England Journal of Medicine, because the participants had not recruited at random; the results have been revised downward.
So what to choose in the avalanche of published studies every day?
Ioannidis recommends asking the following questions: Is it an isolated study or does it strengthen existing jobs? Is the sample small or big? Is it a random experience? Who financed? Are researchers transparent? the adoption of treatments at best ineffective and, at worst, harmful.
In his book "Ending Medical Reversal", Vinayak Prasad and Adam Cifu link terrible examples of practices adopted on the basis of studies that were invalidated years later, such as placement stents in an artery of the brain to reduce the risk of stroke.
Ten years later, a rigorous study showed that the practice increased the risk of stroke.
The solution requires the collective adjustment of common criteria for the actors of research, not only journals: universities, public funding agencies, laboratories.
But all These entities are subject to competition.
"The system does not encourage people to go in the right direction," says Ivan Oransky, co-founder of the Retraction Watch website, which covers withdrawals of scientific articles.
"We want to develop a culture in which we reward transparency".
The media also have their share of responsibility because, according to him, they must better explain to their readers the uncertainties inherent in scientific research, and avoid sensationalism.
"The problem is the endless succession of studies on coffee, chocolate and red wine," he complains. "We have to stop."
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