Julia Roberts Should Read This
by Howard Fienberg
Reports of cancer clusters have soared over the last few decades. Movies like "A Civil Action" and "Erin Brockovich" have laid the template of a murder mystery for news stories about cancer clusters: sympathetic victims, grisly murder weapons (cancers), and the hunt for a villain (e.g., industrial solvents).
For instance, the Richmond Times-Dispatch (Jul. 22) featured a set of articles under the title of "Toxic Burial Ground." It profiled residents like Wade Sprouse, who "may never know for sure what made his wife sick" with osteoporosis, but "suspects it could be the water she drank for many of the 37 years that they lived ... in Rayon Park," and Sheila Wilmoth, who "believes the water she played in as a child is to blame" for her colon cancer and her mother's lung and liver cancers. The not-so-subtly hinted cause of such cancer clusters in Rayon Park? The Defense Supply Center, a Superfund site which may have contaminated the surrounding area with hazardous chemicals over its years of operation. According to the Times-Dispatch, "at least one person in each of Rayon Park's 75 homes has developed cancer."
This is but one example among many. Whether it be a cluster of child leukemia cases in Fallon, Nevada or breast cancer cases among women in Long Island, New York, we can't help but be concerned. Even Congress, led by Senator Hillary Clinton (D-NY), has cried "something must be done!" But what? Are cases of cancer clusters really as straightforward as the plots of dime-store mystery novels?
Cancer is an unfortunately common ailment. The National Cancer Institute estimates that there are more than 8 million cancer cases in the U.S. Cancer is America's second leading killer after heart disease. In fact, almost one in every three Americans dies of cancer. So finding several cases within the same neighborhood or even the same family is not unusual. It should be expected. For one thing, we see more cancer today than in generations past because we live longer, increasing our likelihood of contracting the disease. Nonetheless, we are still not certain what causes most cancers. Researchers continue to grapple with a wide range of possible causes, including heredity, diet, lifestyle and environmental exposure.
So where do cancer clusters fit into this puzzle? They represent the occurrence of a greater than expected number of cases of a particular cancer within a group of people, a geographic area, or a period of time. But clusters may exist more in our minds than in reality. This is sometimes referred to as the 'Texas sharp-shooter effect.' The Texas sharp-shooter shoots at the side of a barn and then walks up and artfully paints a bull's-eye around his shots large enough that all of them fall inside it. People have a natural tendency to encircle groups of events into clusters, but while drawing a circle around a block where three kids have cancer may show a cluster, drawing the circle around the town as a whole shows none. Humans are programmed to see patterns which do not necessarily exist, making it hard for us to comprehend or accept random chance. Flipping a coin a hundred times does not guarantee an equal distribution of heads and tails, but most people become suspicious if the results deviate greatly from that pattern.
For an example we can turn to another supposed cancer cluster. Three male co-workers in Albuquerque contracted breast cancer, a disease that is very rare in men. The lawyer for two of the men told ABC News' Good Morning America (Jun. 4) that "you're more likely to win the power ball lottery two days in a row than find [three] men... in the same workplace with breast cancer, statistically." Unless winners are disqualified from future games, however, odds on winning the power ball lottery should be independent from game to game, person to person.
Diseases work the same way. The chances of each man getting breast cancer may be entirely independent of one another. We cannot expect cancer cases to be equally distributed across all demographics and locations, anymore than we can expect them to all fall in, say, Los Angeles. Daniel Kahneman and Amos Tversky, back in 1971, dubbed this problem the Belief in the Law of Small Numbers - the assumption that the pattern of a large population will be replicated in all its subsets.
A lot of cancer clusters are just random noise. Many of them do not encompass enough cases to allow useful conclusions. Even when they do, statistical analysis often finds no significant increase in disease incidence.
Proven cause and effect in cancer clusters is uncommon. Reasonably conclusive proof can usually only be found in cases where chemical exposure was extremely high, the disease is extremely rare, the population does not move much and is of an age not usually affected by the cancer. In the hundreds of investigations of U.S. residential cancer clusters, no conclusive underlying environmental cause has been unearthed. This brings us back to Rayon Park. Data collection has been conducted primarily by local residents. Neither the local residents nor the Times-Dispatch has given much thought to the locals' lifestyles, which could more easily explain some of the cancer cases (e.g., lung and throat cancer cases might be linked to heavy smoking). And what about the age range of the cases? Many of the paper's anecdotes focus on older sufferers, who are much more likely to contract cancer than younger people.
The Times-Dispatch is not an expert on these diseases, nor are the people who suffer from them. Their assumptions could prove harmful to the community when it tries to respond to the cries of "something must be done." No one yet knows if that "something" could be a massive and immediate clean-up of chemicals or massive changes in how residents live their lives. In fact, the requisite "something" could be nothing at all. News stories like those from the Times-Dispatch provide for compelling narratives. Unfortunately, all the emotional anecdotes of innocent suffering do not add up to scientific data. An actual scientific study of Rayon Park is due shortly. But given the Times-Dispatch's demonstrated skills as a Virginia sharp-shooter, combined with continued public misconceptions of cancer clusters, we are unlikely to hit the mark any time soon.
HOWARD FIENBERG is research analyst at the Statistical Assessment Service (STATS), a nonprofit nonpartisan think tank in Washington, D.C.
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