Okay, without looking at Gig's books and articles, trying to do it off
the top of my head:
.93x13=12.09 (12 out of 13 is good)
.86=(1000-13)=848.82 (849 out of 987 means 138 false alarms for every 12
hits).
So, the probability of actually having Alzhiemer's based on a positive
test here is only 12/138=8.69%
Is that right?
Now, that sounds bad, like Claudia said, but for any low-probability
event like Alzheimers, you always going to have way more false alarms
than hits. It's the same for HIV and breast cancer tests as well.
Chris
===============
[email protected] wrote:
For those of you who are Gerd Gigerenzer fans (and who isn't these days),
here's an exercise for the reader involving a new screening test for
Alzheimer's. Actually, feeling that one never knows when it will strike,
it's just a cheap trick to get you to check my own calculations.
There's a new BMJ report of a self-administered test for Alzheimer's.
Takes only 5 minutes. The authors conclude "It is a powerful and valid
screening test for the detection of Alzheimer's disease". Wow!
They report sensitivity of 93% [probability of correctly detecting
Alzheimer's] and specificity of 86% [probability of correctly rejecting
diagnosis of Alzheimer's].
An accompanying editorial helpfully notes that the prevalence of dementia
[which would mostly be Alzheimer's] is 13 per 1000 in people aged 65-69.
Questions:
1) Using this "powerful" test, for every patient correctly identified as
having Alzheimer's, how many patients will be incorrectly so identified
with this devastating diagnosis?
2) Do you think this test is as useful as the authors claim?
The first person to correctly respond will receive a free orientation to
time and place.
Sources:
Article:
Brown, J. et al (2009). Self administered cognitive screening test (TYM)
for detection of Alzheimer's disease: cross sectional study. BMJ, 338: b
2030 [on-line first]
Free at http://www.bmj.com/cgi/reprint/338/jun08_3/b2030
Editorial:
Nicholl, C. (2009). Diagnosis of dementia. BMJ 338: b1176.
You get a sufficient peek at it, plus, despite its warning that payment
is required, free access to the rest when you click on "full text of this
article" at
http://www.bmj.com/cgi/content/extract/338/jun08_3/b1176?papetoc
Stephen
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Stephen L. Black, Ph.D.
Professor of Psychology, Emeritus
Bishop's University e-mail: [email protected]
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