Allen Esterson wrote:
> Ben Goldacre asks in relation to the recent study of screening for prostate
> cancer:
> "Why would the American and the Australian journalists say something
> completely different to the British ones, about the very same evidence?"
> http://tinyurl.com/c32cw9
>
>
Good article. He presents the frequencies rather than the percentages
(which are nearly always misleading for low-probability events such as
cancer). Here they are:
"the study took over 160,000 men between the ages of 55 and 69 and
randomly assigned them either to get PSA screening, or to be left alone.
The differences were marginal. Yes, there were 20% fewer deaths in the
screening group. What does that mean in terms of real people, in real
numbers you can understand, not percentages? 1410 men would need to be
screened to prevent one death. For each death prevented, 48 people would
need to be treated: and prostate cancer treatment has a high risk of
very serious side effects like impotence and incontinence."
To answer his question, I suspect the answer is that PSA screening is
not routine in the UK, but it is in the US (I do not know about
Australia). As a result, many in the UK feel that this means their
National Health System isn't taking as good care of them but, as the
results show, universal PSA may be nearly a waste of money.
Indeed, it is the questionable use of the PSA test that Gerd Gigerenzer
uses to set up his piece (which I have shilled for several time before)
on Statistical Illiteracy (e-copies still available to anyone who writes
me off-line!). Here's what he says about PSA:
In a 2007 campaign advertisement, former New York City mayor Rudy
Giuliani said, ''I had prostate cancer, 5, 6 years ago. My chance of
surviving prostate cancer---and thank God, I was cured of it---in
the United States? Eighty-two percent. My chance of surviving
prostate cancer in England? Only 44 percent under socialized
medicine'' (Dobbs, 2007). For Giuliani, these health statistics
meant that he was lucky to be living in New York and not in York,
since his chances of surviving prostate cancer appeared to be twice
as high. This was big news. As we will explain, it was also a big
mistake. High-profile politicians are not the only ones who do not
understand health statistics or misuse them....
Giuliani apparently used data from the year 2000, when 49 British
men per 100,000 were diagnosed with prostate cancer, of which 28
died within 5 years---about 44%. Using a similar approach, he cited
a corresponding 82% 5-year survival rate in the United States,
suggesting that Americans with prostate cancer were twice as likely
to survive as their British counterparts. Giuliani's numbers,
however, are meaningless for making comparisons across groups of
people that differ dramatically in how the diagnosis is made. In the
United States, most prostate cancer is detected by screening for
prostate-specific antigens (PSA), while in the United Kingdom, most
is diagnosed by symptoms. The bottom line is that to learn which
country is doing better, you need to compare mortality rates....
Are American men half as likely to die from prostate cancer as
British men are? The answer is no; the risk is about the same: About
26 prostate cancer deaths per 100,000 American men versus 27 per
100,000 in Britain (Shibata & Whittemore, 2001).
From "Helping Doctors and Patients Make Sense of Health Statistics"
by Gerd Gigerenzer, Wolfgang Gaissmaier, Elke Kurz-Milcke, Lisa M.
Schwartz, and Steven Woloshin
/Psychological Science in the Public Interest/, 2008, vol 8, no. 2
Chris
--
Christopher D. Green
Department of Psychology
York University
Toronto, ON M3J 1P3
Canada
416-736-2100 ex. 66164
[email protected]
http://www.yorku.ca/christo/
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