Cancer and Statistical Illusion
The cover of this month’s Wired promises "The Truth About Cancer" but the article inside is a tissue of misleading statistics and faulty logic. The article begins with fancy graphics telling us "If we find cancer early, 90 percent survive" but "If we find cancer late, 10 percent survive." And this:
Find the disease early "and the odds of survival approach 90 percent…This reality would seem to make a plain case for shifting resources toward patients with a 90 percent, rather than a 10 or 20 percent, chance of survival."
Thus, the opening block of text commands, "Scientists should stop trying to cure cancer and start focusing on finding it early. It’s the smart way to cheat death."
The fallacy in all of this is painfully easy to spot. If we measure survival, which these studies do, with a 5 or 10 year survival rate then obviously people whose cancers are detected early will survival longer than people whose cancers are detected late.
The key question is whether people who are treated early survive longer than people whose cancers are detected early but who are not treated. In Thomas Goetz’s long article there is not a single piece of evidence which demonstrates that this is true. Indeed, quite the opposite. About 9 pages into the article, after the jump, we find this about CT scans for lung cancer:
As with the Action Project, these studies found that, yes, CT scans detected a huge number of early cancers–10 times as many as they would expect to find without scanning. In that regard, the scans did their job as a screening test. And as expected, the number of surgeries based on those diagnoses jumped. But when Bach looked at the resulting mortality rates, he found essentially no difference between those who received a CT scan and those who had not. Despite the additional surgeries, just as many people were dying as before.
Nowhere does the author mentions that this finding invalidates just about everything he has told us in the first eight pages.
Addendum 1 : Do note that I have nothing against early detection and I am not claiming that it never works. My problem is with misleading statistical analysis.
Addendum 2: Careful readers will note that this is an almost perfect example of the economicitis fallacy that I blogged about late last year.