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It takes training to focus on a single co-variation. 2x 2 tables are
"abnormal" in the sense that we do not naturally see single
co-variation.
It is for this reason that it is up to those of us who want to
improve on
the use of 2x 2 tables or likelihood ratios, to make them easy to
use and
make them part of the "habits needed to be a doctor".
Unfortunately statistical manipulation of multiple co-variation is
extremely complex. Furthermore, such modelling is likely to remain
as
academic interest for a while yet - at least as long as it remains
slower
than a GP noticing (almost instantly) that a child is "sick".
------------------------------------------------------------------------
I'm not sure it's a good idea to focus on single co-variation. It
makes more
sense to try to use all relevant variables when making a judgment
(or a
decision.) Multivariate statistical models are complex. There are
at least
some that have proven to be good predictors of specific clinical
outcomes.
Although it takes a lot of effort to construct such a model, using
such a
model is only slightly slower than making an intuitive judgment. If
such a
model is more accurate than intuitive judgment, and it is important
to judge
the outcome in question to make a decision, maybe physicians should
learn to
use such models.
-------
Adding to Roy's last point, some literature does suggest that
statistical models are superior to clinical method (see Dawes RM et al.
Clinical vs. actuarial judgement. Science 1989;243:1668-1674)
regards
ben
Benjamin Djulbegovic, MD,PhD
Associate Professor of Medicine
H. Lee Moffitt Cancer Center & Research Institute
at the University of South Florida
Division of Blood and Bone Marrow Transplant
12902 Magnolia Drive
Tampa, FL 33612
Editor: Evidence-based Oncology
http://www.harcourt-international.com/journals/ebon/
e-mail:[log in to unmask]
http://www.hsc.usf.edu/~bdjulbeg/
phone:(813)979-7202
fax:(813)979-3071
> -----Original Message-----
> From: Roy Poses [SMTP:[log in to unmask]]
> Sent: Wednesday, April 05, 2000 9:37 AM
> To: Evidence Based Health List
> Subject: daunting 2x2 tables (fwd)
>
>
>
> ---------------------------------------------------------
> ROY M. POSES MD
> BROWN UNIVERSITY CENTER FOR PRIMARY CARE AND PREVENTION
> MEMORIAL HOSPITAL OF RI
> 111 BREWSTER ST.
> PAWTUCKET, RI 02860
> USA
> 401 729-2383
> FAX: 401 729-2494
> [log in to unmask]
> ----------------------------Original message----------------------------
> 2x2 tables are a numeric representation of single covariation. For
> example
> the relationship of the pulse to the outcome of an asthma attack can be
> analysed by 2x2 tables. Naturalist epistemologists would argue that such
> representations are abnormal for humans. They argue humans see the world
> in terms of multiple co-variation rather than single covariation. In
> other
> words a GP will see a child is "sick and needing to be admitted", but will
> not see a particular sign in that child. I did an audit of asthma care in
> our area and found that the majority of GPs referred patients to hospital
> who were in the appropriate range of severity for asthma (no admitted
> patients were "well"), but when we looked at the GP notes we found that
> there were many pertinent details not recorded - for example the pulse was
> rarely recorded. Now when we asked GPs about the pulse, everyone claimed
> the pulse is an important sign to record. Perhaps these results reflects
> a
> universal inability by gps to record single items of data. Indeed we found
> individual data items were recorded more often when we introduced an
> asthma
> data recording form - a change from 48% to 68% of patients with acute
> asthma had their pulse recorded. However many GPs commented that "we dont
> go by the pulse" or "I judge what to do by how sick the child looks". So I
> guess the influence of observation at a level of multiple covariation is
> still present.
> ----------------------------------------------------------------------
>
> Studies in judgment and decision psychology suggest that people have poor
> insight about the factors they actually use to make judgments. Clare
> Harries'
> studies of physicians in the UK showed similar results: the variables
> doctors
> say are important to making a particular judgment are not the same as the
> variables they seem to use when making judgments for individual cases. (I
> can
> find the reference if anyone wishes.) So I guess I'm not surprised
>
> Another question is whether these physicians' judgments were accurate, and
> whether they lead to good decision making. If no admitted patients were
> well,
> were any patients who were not admitted sick?
>
>
>
>
>
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