How to use medical evidence I: the nature of the evidence

October 28, 2008  |  General

This is the first of several posts about how to read the medical news. How can a non-physician interpret the latest breathless bulletin about some new breakthrough? Are there commonsensical tests one can apply to the story to see how valid it might be? There are. One useful notion is that we have ways of grading the validity of different sorts of evidence. I’ll get to those in later posts. First, it is useful to know where we’ve come from.

Medicine is not totally a science. It makes use of science and struggles as best it can to adhere to scientific principles, but medical knowledge, perhaps because it involves humans, will always contain an element of human fallibility. This is more true of some aspects of medicine than others. For example, we understand the science of how a damaged bone heals far better than we understand the science of how a damaged mind does. Medicine is occasionally more or less science-inspired intuition. A good share of the frustration non-physicians have with medicine stems from this misunderstanding of what medicine fundamentally is. Yet in spite of all the inherent vagaries of medical practice, the fact remains: we can know things, there are tools to help us do this, and these tools are based on the scientific method.

Throughout most of its history medicine had no scientific footing at all, and for centuries it was as much a branch of philosophy as it was anything else. The theories of how the body worked were fanciful notions with little or no basis in observed reality. The Scientific Revolution changed that. Ever since William Harvey decided in the early seventeenth century to test thousand-year-old teachings of how the blood circulated by actually looking at what happened, the principle of experimental observation has been a foundation of medical science. Although the accepted ideas of disease causation remained outlandish to our modern eyes for many years after Harvey’s pioneering experiments, the world of medicine had still changed profoundly and permanently. Henceforth the best physicians would actually test to see if their theories had any basis in observed reality.

For Harvey and the thousands of medical researchers who came after him, if they had a theory about how the body worked, they tested it with experiments. But it is also important to understand we can use sound principles of scientific observation even when we have no idea, no theories, about why things happen. For example, we can compare two ways of treating a disease though we have no notion of what causes the disease. It is often the results of such naïve observations which lead to theories of disease causation, theories which can then be tested with further observations. But we can still reach some useful conclusions about what works and what does not work without having any understanding at all of the reason why.

The vital distinction to make is this: although we can have good observations without underlying theories, we cannot have useful theories that are not based upon some kind of valid observations. What does this principle mean for today’s parents? From the outset, parents should demand of anybody making a claim about how children’s bodies work that they show good evidence in support of their theories. And good evidence can be hard to find.

The physicians of Roman times, whose theories Harvey disproved, saw no need to provide any experimental evidence at all. These days parents hear conflicting evidence everywhere; all the partisans in these debates have evidence. The problem is to decide among the conflicting claims whose evidence makes sense and whose does not. Can parents do that? Again, without the need to make a biostatistician of anyone, I believe they can. What they need to learn, and then practice, is how to look critically at medical claims.

The first step is to understand that not all evidence is equal, even if the people collecting it wear a long, white coat and have degrees after their names. Like good detectives, we need to weigh the validity of different kinds of evidence, and medical scientists have established a hierarchy of kinds of evidence to help decide which is the most likely to be correct. The hierarchy runs from least reliable to most likely to be right. Parents may be surprised to learn the least reliable category is expert opinion, what an individual expert or even a committee of experts say is correct. Next in believability comes various forms of what are called uncontrolled studies, and most reliable are what are called randomized, controlled trials (RCTs).

You will read about the first of these, expert opinion, in the next post.

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