Tuesday, December 7, 2010

Of P values, power, tobacco and cell phones

Like everything else political, science, as written in the tobacco playbook, is being used for divisive purposes. A new study from Denmark, as reported by WebMD, indicates that exposure to cell phones in utero may influence the child's behavior. The authors are appropriately tempered in their conclusions and subsequent recommendations, responsibly acknowledging the obvious limitations of their research. Yet, to paraphrase one of the authors, what is the harm in asking a pregnant woman to keep her cell phone away from her uterus? It is at most a slight inconvenience. This sentiment is echoed by Devra Davis, the author of Disconnect, a book summarizing what we know about human health effects of microwave radiation and cellular technology, in her comments to WebMD, wherein she plainly echoes the authors to say that the current study was not perfect. Yet, she urges precaution until we know more about the potential ill effects of cell phones, particularly on the most vulnerable in our population.

Now, I have blogged before about Devra Davis and about my sense that her messages of caution are less than popular even with those who consider themselves to be scientifically skeptical. Perhaps especially with those who consider themselves to be scientifically skeptical. It seems to me that we have gone a little bit overboard on this critical thinking idea, where we demand data on harms to reach the same level of evidentiary standards as salutary effects. This is a fallacy. It is a convenient fallacy, to be sure. In fact, what many people may not realize is that it is much more than an accidental byproduct of our skepticism. No, it is a strategy, as discussed in Davis's book The Secret History of the War on Cancer deliberately developed and skillfully executed by the tobacco industry to cast doubt for decades on the harms of smoking. Look how well it has done -- 80 years of knowing of cancer risks associated with smoking and nearly 50 years after the Surgeon General's report on the dangers of smoking, we are still looking to mass screening for diseases caused by tobacco rather than mass cessation efforts to diminish its impact. This incredibly effective and durable strategy relies on scientific data generated by men with loud reputations funded by the tobacco companies to demand unequivocal causality between the exposure and the outcome. So, inherently, it demands bodies in the street to show unequivocal links.

Predictably, now the cell phone industry is using the same strategy. Here are a couple of quotes from the WebMD article, which I personally find less than amusing:

John Walls, vice president of public affairs at CTIA-The Wireless Association, a trade group representing the wireless industry, tells WebMD that his group “stands behind the research review by independent and renowned public health agencies around the world which states that there are no known adverse health effects associated with using wireless devices.”
Jeff Stier, a senior fellow at the National Center for Public Policy Research, a conservative think tank, says that the new study is full of holes. “For starters, self-reporting of cell phone use makes it impossible to assign any meaning to the exposure,” he says.
“Different phones give off different exposures, and even those who were reported to be not exposed, probably had significant environmental exposure, rendering the study only slightly more than amusing,” he tells WebMD in an email.

Surprise! The science is just not there! So, let's party, right?

Well, wrong. In epidemiology we set a very high bar for avoiding a false positive, and a much lower bar for a false negative. You have all heard of the much misunderstood p value. What this number represents is exactly the probability of chance giving us the result of the magnitude observed or one of a greater magnitude under the circumstances of no real association. By convention (whim?) we usually set the threshold for significance at <0.05, meaning that there is a <5% of obtaining the given result or one of greater magnitude by chance if there is no actual association. This clearly stacks the deck against calling an association positive. In contradistinction, we are much more willing to obtain a false negative, as illustrated by the conventional power of a study to detect a difference where a true difference exists at only 80%. So, colloquially, we want to be 95% sure that a positive result is not a false positive association, yet only 80% sure that a negative result is not a false negative. You see the difference? We are clearly more keen to discard associations that do not really exist than to identify them.  To an untrained eye the difference may be subtle, but it creates a world of philosophical difference in how we approach data.

And it is exactly these in some ways arbitrarily accepted standards of our science that are corrupted to sell false certainty of the negative associations. I have written about my thoughts on proof of harm vs. benefit in prior posts. I firmly believe that it is our interest in being more certain that an association is not spurious at the expense of being more willing to discard associations that are true that is facilitating our national blindness to many potentially adverse health effects from stuff around us. I am convinced that causation of harm needs to be viewed with greater caution and therefore with greater scientific permissiveness. If we do not adopt this precautionary principle, used by the EU liberally in their environmental policies, we will continue to see even more piles of bodies in the streets -- just look at tobacco. This is especially true when all there is is a risk with no benefit (tobacco), or when a simple behavior change can eliminate any concern for an adverse outcome (cell phones).

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