I understand the principle here, and this may have already been addressed, but is the number of 65-69 year olds per thousand with dementia the appropriate comparison group? Unless we are going to deploy this test willy-nilly on all 65-69 year olds (for example, when they show up to renew their driver's license or each time they go to the doctor along with their weight and bp), it seems that someone taking a screening test for Alzheimer's is probably already in a select population of individuals who are showing some symptoms or evidence of a disorder. We might overstate the number of false positives by ignoring that fact. Note that most of the examples given by Gigerenzer are cases where symptom-free tests are being recommended (preventative colonoscopies or mammograms for everyone over a certain age).
I think the test itself is also pretty humorous. With its questions like, "Who is the Prime Minister?" "In what year did the 1st World War start?", I think its secret purpose might be to declare the average USA citizen demented (with possibly an as yet unknown epidemic of dementia among the young). Rick Dr. Rick Froman, Chair Division of Humanities and Social Sciences Professor of Psychology Box 3055 John Brown University 2000 W. University Siloam Springs, AR 72761 [email protected] (479)524-7295 http://tinyurl.com/DrFroman -----Original Message----- From: Christopher D. Green [mailto:[email protected]] Sent: Thursday, June 11, 2009 12:03 PM To: Teaching in the Psychological Sciences (TIPS) Subject: Re: [tips] Gigerenzer alert: an exercise 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 > ----------------------------------------------------------------- > Stephen L. Black, Ph.D. > Professor of Psychology, Emeritus > Bishop's University e-mail: [email protected] > 2600 College St. > Sherbrooke QC J1M 1Z7 > Canada > > Subscribe to discussion list (TIPS) for the teaching of > psychology at http://flightline.highline.edu/sfrantz/tips/ > ----------------------------------------------------------------------- > > --- > To make changes to your subscription contact: > > Bill Southerly ([email protected]) > > --- To make changes to your subscription contact: Bill Southerly ([email protected]) --- To make changes to your subscription contact: Bill Southerly ([email protected])
