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Harvard Commentaries
Harvard Commentaries
Reviewed by the Faculty of Harvard Medical School

What Your Doctor Is Saying What Your Doctor Is Saying

Common Versus Rare Conditions

October 10, 2013

By Robert H. Shmerling M.D.

Beth Israel Deaconess Medical Center

Healthy Lifestyle
What Your Doctor Is Saying
Common Versus Rare Conditions
Common Versus Rare Conditions
There's a saying among medical trainees that "when your hear hoof beats, think horses, not zebras." All other things being equal, something common is more likely to explain an ache, pain or other symptom than something rare.
Harvard Medical School Commentary

There's a saying among medical trainees that "when your hear hoof beats, think horses, not zebras." In other words, something common is more likely to explain an ache, pain or other symptom than something rare. Of course, if the situation changes, what originally seems rare may be worth considering, and the rare may become more likely; if you find yourself in Africa, zebras may be a reasonable explanation for the hoof beats you hear. Although this may seem obvious, this statistical principle — that a common problem or condition is far more likely to occur than something rare — often affects the way your doctor approaches your medical evaluation.

What's Common Is Likely

Flip a coin. How will it land? We generally think of the results as heads or tails, and the vast majority of the time the result is one of the two, but the coin could land on its edge. Because the chances of that happening are tiny, it's usually not even worth considering. Your doctor does the same thing when evaluating your symptoms, findings of a physical examination and test results: Common things occur so much more often than rare things that it's usually unhelpful to focus on the exceptionally rare things, at least at the start.

Assuming that the situation is not an emergency, your doctor will probably focus on common things even though it's often the rare (and awful) things patients often worry about the most. If you have a headache, a migraine or tension headache are high on the list of possible explanations. On the other hand, in certain situations (such as headache with fever and confusion) even a rare condition, such as meningitis, can become more likely.

Because common things occur commonly, two common occurrences may happen close in time but be unrelated. For example, imagine that you take a medication and several hours later you have a headache. Both events are common and could easily have occurred together just by coincidence, but the timing may convince you that the headache is a side effect of the medicine. If you swear off that medicine forever because of presumed side effects, the benefits of that medicine will be lost. Although headaches may occur as a medication side effect, for many medicines, it is statistically much more likely that the timing was due to chance.

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Dealing With the Common and the Rare

For many people who have symptoms, it is natural to worry about the worst thing it could be. And it’s common to remember the most dramatic or unexpected situations rather than the routine. If you knew someone who died from a brain tumor and his or her problems began with headaches, it may be hard not to worry when you or a loved one complains of headaches. And that's true even if you know that the vast majority of headaches are not due to serious disease.

Health care professionals are often taught that “uncommon symptoms of a common disease are more common than common symptoms of an uncommon disease.” This means that because common conditions occur so often, they sometimes have unusual features; even so, a common disease is more likely to be the culprit than an absolutely classic case of a rare disease.

As an example, consider a person who sees a doctor during the middle of flu season for a fever, headache and a stiff neck. While these are the classic symptoms of bacterial meningitis, the flu is more likely. There may still be a role for tests to rule out rarer and more serious illnesses, but remembering that “common things are common” is a good starting point in most situations.

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Occam's Razor

William of Occam was a philosopher and theologian during the 14th century, not a medical person. So it may seem surprising that one of his ideas has influenced modern medical thinking. Your doctor has probably heard of Occam's (or Ockham's) razor, the idea that when one simple explanation will suffice, it is more likely to be true than a complicated explanation or multiple explanations. In medicine, it means that if a single simple diagnosis adequately explains the situation well, that diagnosis is statistically more likely than two or three diagnoses which, when combined, might also explain the situation well.

There are important exceptions to Occam’s razor, however. Sometimes, people are unlucky and although the odds are against it, they have two problems at the same time rather than one. Another example is that certain conditions markedly increase the chances of having more than one problem at once. AIDS is a good example, in which the immune system is working so poorly that multiple infections may develop around the same time; to make matters even worse, people with AIDS may develop one or more types of cancer as well. Among persons with AIDS, there are often exceptions to Occam's razor.

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The Bottom Line

Although we may wish otherwise, uncertainty is common when it comes to medical issues. There is likelihood, risk, and probability at play in almost every medical situation. It is typically fruitless, expensive, uncomfortable and sometimes risky to pursue every rare possibility that might be responsible for your symptoms. Focusing on what occurs commonly may explain why your doctor suggests one test but not another. We all worry about the rare, awful things; fortunately, most of the time we actually experience the common and the benign. Good doctors know a lot about the common and rare conditions that could account for your symptoms; but the best doctors know when to move past the common and focus on the rare. It may be reassuring to hear from your doctor that something minor or common is probably going on, but if you are worried about something serious or rare, be sure to let him or her know.

When you ask, “What's causing my headaches?” there's a reason your doctor’s answer may not include the things you are most worried about. When he or she starts talking about what is likely or what is unlikely, here's what your doctor is really saying: In the face of uncertainty, knowing what is common is usually the best place to start.

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Robert H. Shmerling, M.D. is associate physician at Beth Israel Deaconess Medical Center and associate professor at Harvard Medical School. He has been a practicing rheumatologist for over 20 years at Beth Israel Deaconess Medical Center. He is an active teacher in the Internal Medicine Residency Program, serving as the Robinson Firm Chief. He is also a teacher in the Rheumatology Fellowship Program.

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