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Autism and mammography: Two stories of statistical confusion

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whiteboard with figures on itDENVER—There was substantial public outcry last year when new recommendations for mammograms came out suggesting that women could wait until age 50 to start breast cancer screening—and then only get screened every other year. Figures in support of the new policy were bandied about in the news and in doctors' offices, regarding lives saved from breast cancer (eight women per 1,000) and false positive results (2,250 per 1,000 women). But in spite of the new recommendations, many women and clinicians were unsure about forgoing the additional screenings called for under the old policy if there was hope of saving even one life.


The U.S. Preventive Services Task Force, which made the recommendations (that women ages 50 to 69 get mammograms every other year), based their decision on analyses of eight randomized controlled trials. Although eight might not sound like a lot of studies for a disease that affects so many women (about one in eight will be diagnosed with breast cancer at some point in her lifetime), when compared with the larger literature on disease screening, "that is an embarrassment of randomized controlled trials," Ned Calonge, of the Colorado Department of Public Health and Environment, said here Tuesday at the American Public Health Association annual meeting. And by embarrassment, he means "a plethora of data," he explained.


The one in eight statistic can be confusing, Calonge said, because most women who get breast cancer do not die from it. In fact, from a baseline of 1,000 women who never have a single mammogram, 30 will die from breast cancer (if followed from age 40 onward). And that, Calonge pointed out, is already a small number to try to whittle down. 

 

If screening is bumped from biannual to annual for women starting at age 40, it would save almost one additional breast cancer death per 1,000 women, he noted.


The reason that more screening earlier does not translate into many more lives saved, which has been borne out through studies and meta-analyses, is that "as you get older, the test itself gets better," Calonge said. The shift in breast composition results in fewer false positives as women reach their 60s, which is "when you really need to push screening—because it becomes a very good test at that age."


And with more tests comes more potential for harm caused by stress from false positive and unnecessary invasive procedures. Just doubling the number of mammograms performed, Calonge noted, would also double the number of false positives and unnecessary biopsies. (One thousand women getting screened every year between the ages of 40 and 69 will produce some 2,250 false positive results and 158 unnecessary biopsies over those 30,000 mammograms.)


The bottom line, he noted, is what so many public health experts, physicians and journalists offered last November: Women in their 40s "should weigh the risks and benefits" and talk with their doctors about whether they want to start screening for breast cancer early, Calonge said.


Autism

Misapprehension of statistics and scientific process has been even more apparent in the misunderstandings surrounding vaccines and the onset of autism.


Given the age at which children receive immunizations and that at which many cases of regressional autism manifest themselves (in which a seemingly normally developing child suddenly loses much of the ability to communicate as well as other acquired functions), "by chance alone" there will be a lot of children who regress at some point after getting their scheduled vaccines, Daniel Salmon, a vaccine safety specialist at the U.S. Department of Health and Human Services (HHS), said here on Tuesday.


As he pointed out, however, "temporality is insufficient to show causality." But underlying—and perhaps highlighted by—this "logical fallacy," he explained, is a frequent hang-up of science communication: the devil is in the details, and the details can be complicated (and not too catchy) to explain.


When Jenny McCarthy, an advocate of the vaccine-autism link, goes on CNN's Larry King Live and says, "'Vaccines cause autism,' that's a very clear, simple message," Salmon noted.


Most respected scientific bodies, however, are not prone to such blanket statements. In a 2004 report essentially dismissing the assertion that vaccines cause autism, the Institute of Medicine (IOM) was notably more measured than McCarthy, concluding that "based on this body of evidence, the committee concludes that the evidence favors a rejection of a causal relationship between thimerosal-containing vaccines and autism…" The report also included appropriate caveats about the limitations of the available data, which many in the anti-vaccine community took as evidence that there was in fact evidence for a link.


Much of the rallying around vaccines (and/or their thimerosal additive) and autism has centered on powerful stories about children who developed regressional autism shortly after receiving immunization. But "the anecdote is not data—though it often seems that way" in the public debate about autism's causes, Salmon said. And likewise, correlations are not the same as causation. All kinds of outside factors, from recommended vaccines to the size of the internet, can be plotted to match the rising curve of autism rates, he noted, underscoring that "I'm not suggesting that the internet causes autism."


Despite the dearth of durable data showing that vaccines can induce autism, some 25 percent of parents in the U.S. still believe them to be a possible cause (a statistic which is "really quite remarkable," Salmon said with a bit of distress).


And the cautious nature of science is unlikely to be able to dispel belief in the link any time soon. "It's exceedingly difficult—and some in epidemiology would say impossible—to prove a negative," Salmon said.

Image credit: Flickr

The views expressed are those of the author and are not necessarily those of Scientific American.

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