Understanding risk statistics about breast cancer screening

An article published in the British Medical Journal (BMJ) today says a US charity “overstates the benefit of mammography and ignores harms altogether.” The charity’s questionable claim is that early detection is the key to surviving breast cancer and to support this, it cites a five-year survival rate of 98% when breast cancer is caught early, and 23% when it’s not.

We’re not interested in judging the charity’s actions or intentions but would like discuss the importance of statistical literacy in communicating medical risks.

There are two critical claims in the argument presented by the experts in the BMJ report – that routine breast screening results in high false positive diagnoses and that five-year survival rates are biased. It’s necessary to understand them both to be able to judge whether the statistics quoted by the charity are misleading.

The Conversation

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10 responses to “Understanding risk statistics about breast cancer screening

  1. Ed, do academics and researchers in Australia have to be registered?

    I saw .edu on the Conversation’s URL, so I checked the ‘about’ link and saw this:

    “Our team of professional editors work with more than 3,400 registered academics and researchers.”

    Are they considered dangerous?

  2. It is interesting to note that breast cancer statistics and screening requirements are only accepted if they result in more screening and testing. When it was suggested that mammograms and pap smears could be done every three years in certain groups, women’s groups threw a fit until the annual requirement was reinstated. It’s not really about science…….

    • I agree, Mr. Check, that it isn’t about science. However, the truth isn’t clear. As government takes over health care, they are shunning the cost of their previously loved “preventative care.” Any of their pronouncements on testing should not be trusted.

      I liked what the article said about reporting actual numbers vs. probabilities; I’ve harped about that before.

  3. Gamecock:
    I agree that the truth about cancer and prevention thereof is not clear at all. My point was the cherry-picking of outcomes that one follows. The preventative care aspect of the government’s health care plan is problematic in general. “Free” testing results in increased costs and testing of people who do not need tested just to increase income. Someone probably just realized that are back-pedaling on the promises. Or if I was really jaded, I’d say they planned it that way….

  4. GoneWithTheWind

    There is another factor in calculating the “benefit” of early detection that is misleading. Take two people with similar cancers. One is detected by chance a year or so before it would have been detected by symptoms. That person is treated with standard treatment and eventually dies from the cancer after living three years since detection. The second person develops the same cancer and one day, a year after it could have been detected, schedules a doctors appointment because of pain and other symptoms. The patient is treated with standard treatment and dies after two years. What is the difference? The difference is that the early detection will be credited with giving the patient an extra year of life but in fact the outcomes were identical.

    • You presume the one detected early doesn’t survive any longer. Bogus.

      Breast cancer is known to spread over time. Catching it earlier will reduce the spread. Just how much can be debated, but there is no doubt that earlier is better.

    • GC, true, but Scarlet here is correct that it is a confounding factor that influences the statistics.

  5. From breastcancer.com:

    “About 20% of women are diagnosed at the very earliest stage (before the cancer has moved beyond a breast duct or ducts to invade surrounding breast tissue). Another 70% learn of their cancer while it’s still contained within the breast or draining lymph nodes. Both groups have good prospects of avoiding metastasis altogether.”

    Obviously, not checking your boobs and just checking your underarm nodes is statistically adequate. But if I were a woman, I’d want it found earlier. Screw the statistics.

  6. A great article.

    One factor left out of the discussion above is the anguish associated with a false positive. The technician who gave my ex-wife said something was interesting in the x-ray. By the time my wife got home, about a half hour, she convinced herself that she had cancer, was going to die in short order, and that her death upcoming was my fault.

    I consoled my ex-wife to wait for the doctor’s take on the x-ray. There was nothing there according to the doctor. But my wife insisted on extra mammograms and got them.

    Mammograms are cash cows and one has to wonder how much of the insistence upon them is based on the best interest of patients. The hysteria in the political realm is disquieting especially from the Senator from Maryland. .

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