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See Inside Scientific American Volume 306, Issue 5

Readers Respond to "The Coming Mega Drought" and Other Articles

Letters to the editor from the January 2012 issue of Scientific American


SCREENING STATS
During my 30-year practice of diagnostic radiology, I spent many hours educating physicians and surgeons on the importance of false positives and false negatives in the diagnostic process. No diagnostic test is 100 percent accurate. My mantra was always: don’t treat initial test results. Always confirm the diagnosis with other independent data before performing surgery or prescribing pharmaceuticals with serious side effects.

I applaud the general theme of mathematician John Allen Paulos in “Weighing the Positives” [Advances]. First he makes the valid argument that medical tests will be positive for some patients without disease. He then illustrates this with a statistical analysis of mammography on one million patients, resulting in 9,960 false positives. He makes a monumental error, however, in stating, “If the 9,960 healthy people are subjected to harmful treatments ranging from surgery to chemotherapy to radiation, the net benefit of the tests might very well be negative.”

Because mammography, prostate-specific antigen levels and all other initial testing for common cancers are merely screening tests, no patient ever receives definitive treatment for cancer before these tests are confirmed by a biopsy. Cynical health care watchdogs may cite this as excessive testing, but such measures avoid the negative effects of overtreatment that Paulos invokes.
J. G. McCully
via e-mail

PREDICTIVE PREJUDICE
In “The Department of Pre-Crime,” James Vlahos mentions the potential danger of prejudging individuals by using predictive policing techniques but avoids discussion of a more serious potential consequence of such “crime forecasting”: the positive-feedback reinforcement of existing biases to more deeply criminalize certain populations and deepen injustice.

If police are already focusing on and arresting in some neighborhoods over others, feeding information into the machine may result in still greater police presence, more arrests, more predicted crime, still more police presence and still more arrests. If the initial bias is for factors other than actual crime, the result may be the deepening of injustice, not a reduction of crime.
The racial, ethnic and financial divides in crime and justice in the U.S. are well documented. The most obvious examples are in the discrepancies in drug laws, where the use of “crack” cocaine gets far more serious penalties than the powdered version, with the meaningful difference being that crack is used primarily in black communities.

African-Americans are perhaps eight times more likely to be incarcerated than whites. Poor people are much more likely to be convicted and sent to prison than wealthier people. Young people in poorer, nonwhite neighborhoods have a much different experience with respect to the police than whites. They are probably more likely to get a criminal record than their white counterparts in wealthier communities who engage in the same behaviors.

Once into the criminal system, people can lose their right to vote, have their reputations and futures tainted, and have reduced access to jobs. They are, in a sense, trained to continue and pass on a more criminal culture.
Michael Jacob
via e-mail

OVERRATED DOWN UNDER?
Although the gist of the “The Coming Mega Drought” [Forum]—Peter H. Gleick and Matthew Heberger’s essay on the possibility of Australia’s Millennium Drought being repeated in the southwestern U.S.—rings true, the comments praising Australia’s response to its drought need a bit of context. There is unfortunately a political aversion to human reuse of water in Australia. (I have heard a specific put-down: “Would you like to drink poo water?”) The $13.2 billion being spent by the country’s five largest cities to add to desalination capacity is extremely wasteful as the same end can possibly be achieved by treatment and reuse. Desalination is also energy-intensive.
James Fradgley
Wimborne, England

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