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Cutting through the Hype IV: Expert Opinion – the Lowest Level of Evidence

“All truth passes through three stages. First, it is ridiculed. Second, it is violently opposed. Third, it is accepted as being self-evident.” – Arthur Schopenhauer

Evidence-based medicine (EBM) is the idea that medical decisions should be based on the best scientific evidence available.1 This does not sound like a radical idea until you consider that up until very recently the experience and intuition of individual doctors was the law of the land. It is surprising that the term EBM was first used only 30 years ago (1991) and that concerted efforts to bring scientific methodology to bear on clinical decisions began only in the last 50 years or so, and have often been met with resistance.2,3

While the public and news media frequently look to experts and expert opinion as the highest evidence available, all systems of EBM that even consider expert opinion as evidence rank it as the lowest form of evidence. Why is this?

The short answer is that an opinion, even that of a true expert, is only as good as the evidence behind it. Sometimes an expert may provide a very well-reasoned opinion based on a wealth of evidence, including randomized controlled trials in people. However, in many cases, expert opinion is simply an educated guess based on personal anecdotes or animal or correlational studies, all sources of evidence that we know have limited power to predict safety or effectiveness in humans. Sometimes it is simply a statement of belief based on the status quo and culture of a particular field of medicine or science.

One can look at the history of advances in science through the lens of experts being proven wrong again and again. This is in fact what Thomas Kuhn does in his highly influential 1962 book The Structure of Scientific Revolutions. At any particular point in history the majority of individuals in a field, typically led by the majority of experts in that field, hold a worldview or paradigm. That paradigm is challenged by new evidence and, often after a period of argument and verification, the theory supported by the new evidence becomes the new paradigm. Ideas we now take for granted—that the earth revolves around the sun, that handwashing improves surgical safety, that certain illnesses are caused by vitamin deficiency—were all at one point opposed by experts.

The key takeaway here is NOT that all opinions are equal. The opinion of “that one friend of yours” who thinks that masks spread COVID is not just as valid as Anthony Fauci’s opinion that masks stop the spread of COVID. On the other hand, the opinion of your local doctor may be more valid than a best-selling celebrity doctor if it is based on the latest research. Evidence is the great equalizer. The key takeaway is that opinions are only as good as the evidence behind them. Choose the experts you follow wisely, but don’t be afraid to question them and go to the source.


1. Burns PB, Rohrich RJ, Chung KC. The levels of evidence and their role in evidence-based medicine. Plast Reconstr Surg. 2011;128(1):305-310.

2. Sur RL, Dahm P. History of evidence-based medicine. Indian J Urol. 2011;27(4):487-489.

3. Smith R, Rennie D. Evidence based medicine–an oral history. BMJ. 2014;348:g371.


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