Toby,
A charitable view of Science is that it can provide the correct answer to a
question, provided that the question is well formulated.
The problem for the practitioner is to determine which of the many questions
in play is the right one to seek to answer.
Paul
Paul Robinson
GP & Course Organiser
Scarborough
-----Original Message-----
From: [log in to unmask]
[mailto:[log in to unmask]] On Behalf Of Toby
Lipman 7, Collingwood Terrace, Jesmond, Newcastle upon Tyne. Tel
0191-2811060 (home), 0191-2869178 (surgery)
Sent: 27 July 1999 23:35
To: Djulbegovic, Benjamin
Cc: 'N.P. van Duijn'; 'Evidence-based discussion group'
Subject: Re: common sense and the standard of truth...
In message <[log in to unmask]
.edu>, Djulbegovic, Benjamin <[log in to unmask]> writes
>So, you are saying that there is no such a thing as "hierarchy of evidence"
>?
>ben
>
As so often, Dave Sackett had it right when he wrote about "choosing the
best research design for each question" (BMJ 1997;315:1636). The thing
that worries me most about the hierarchy of evidence is the way it
places the meta-analysis of RCTs at the top as though this is therefore
the "best" evidence. It isn't. It's only the most valid experimental
evidence (and if the extent of heterogeneity in various aspects of
trials has been underestimated it might sometimes not even be so
valid...).
The danger is that we might assume that the most important questions are
those that can be answered by evidence which is high in the hierarchy.
So the question "do statins reduce cardiovascular morbidity and
mortality in patients with raised cholesterol" might be thought a more
valid or important question than "do people who eat a Mediterranean diet
have a lower prevalence of ischaemic heart disease than people who eat a
diet high in saturated fat?",or "why do poor people eat a diet high in
saturated fat?", or "what stops people taking exercise?"
Our strategy for reducing cardiovascular disease might therefore, if we
prefer high hierarchy evidence, exclude interventions aimed at
alleviating poverty or encouraging a healthy lifestyle "because there is
little valid evidence that these interventions are effective"
This is to take far too narrow a view, not only of what constitutes
valid and (especially) applicable evidence, but of what health care
consists of.
The main use of hierarchies of evidence, in my view, is as a shorthand
to link recommendations in evidence-based guidelines to the evidence, so
that when you read the guideline you know what class of evidence has
been used. The applicability of the evidence is related to its
usefulness and relevance as much as to its position in the hierarchy.
Thus qualitative evidence that people believe their children are at risk
of developing pneumonia if they don't get antibiotics might be equally
as useful as evidence that antibiotics don't prevent pneumonia in
children with coughs (indeed you might find an attempt to use the latter
very frustrating if you didn't understand the former!)
Toby
--
Toby Lipman
General practitioner, Newcastle upon Tyne
Northern and Yorkshire research training fellow
Tel 0191-2811060 (home), 0191-2437000 (surgery)
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