Dear Jeremy,
There are 2 different purposes to evidence hierachies: (A) a
searching guide and (B) grading of evidence.
The first is used by EBMers to answer specific clinical questions
arising in the care of patients, whereas the latter occurs in
guideline writing.
A. As a searching guide it works something like this:
I have a question about treatment arise in caring for a patient, and
I have (only) a couple of minutes to find the best current evidence.
1. I check if there is a systematic review (I'd use PubMed:Clinical
Queries as it picks up the Cochrane and non-Cochrane reviews). If I
find a systematic review, I still need to check the quality is OK.
2. If there is no systematic review, then I look for individual RCTs
(and focus on the largest well-conducted study first). If an RCT
exists, I then need to appraise the quality. If its no good (e.g, a
small and flawed study) or there are no RCTs then I'd move on to the
next level,
3. If there are no RCTs, then I'd check if there are any cohort
studies. If a cohort study exists, ....
etc
The evidence "hierarchy" is used as a search heuristic, with
alternation between finding evidence at a particular level and
checking the quality of what is found.
B. A guideline writing (who is hopefully also an EBMer, but that
seems pretty rare) would hopefully work through the same process, but
also give a grade to the best evidence found. As the guideline writer
will have more time they might also check the next level of evidence
down the hierarchy. The GRADE group are looking at this, and includes
some issues on combining evidence:
http://www.gradeworkinggroup.org/
Best wishes,
Paul Glasziou
> >>> "Howick,J (pgr)" <[log in to unmask]> 01/04/06 2:33 pm >>>
>Hi,
>
>Happy New Year! I'm a PhD candidate at the London School of Economics
>(LSE) specializing in the Philosophy of Medicine. I am currently
>working
>on a project with Professor Nancy Cartwright on Evidence for Policy
>Evaluations, which may eventually be of interest to the CEBM.
>
>I have a question regarding the CEBM grades of evidence that was not
>answered on the website (http://www.cebm.net/levels_faq.asp). Here is
>my
>question:
>
>How would the CEBM weigh level 2 or 3 evidence with level 1 evidence?
>For example, what if there were many observational studies which
>conflicted with a single well-done RCT - would the results from the
>more
>numerous observational studies outweigh the single RCT?
>
>Please forward this question to whoever can answer it. Cheers!
>
>I look forward to hearing from you soon,
>
>Jeremy Howick
Paul Glasziou
Department of Primary Health Care &
Director, Centre for Evidence-Based Practice, Oxford
ph: 44-1865-227055
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