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I would suggest a pre-planned stratified randomization with analyses within the stratified groups could provide level 1 evidence. This could be called a "subgroup analysis" but would be the equivalent of independent randomized trials being reported as a single trial. All of the usual considerations for bias would have to be considered at the level of the "subgroup" -- adequate sample size, adequate follow-up rates, intention-to-treat analysis, etc. In addition consider whether the effects of other trial activity (including statistical evaluations) could in any way bias the chances of finding and reporting significant differences in the "subgroup". If the randomization was not stratified by the subgrouping, then the clustering of confounding factors along with the "subgroup factor" could nullify the initial reason for randomization. Adjusting for recognized confounding factors cannot exclude bias from unrecognized confounding factors. -------------------------------------- Brian S. Alper, MD, MSPH Editor-in-Chief, DynaMed (www.DynamicMedical.com) Medical Director, EBSCO Publishing 10 Estes St. Ipswich, MA 01938 office (978) 356-6500 extension 749 cell (978) 804-8719 fax (978) 356-6565 home (978) 356-3266 "It only takes a pebble to start an avalanche." -----Original Message----- From: Evidence based health (EBH) [mailto:[log in to unmask]] On Behalf Of Olive Goddard Sent: Wednesday, May 09, 2007 4:14 AM To: [log in to unmask] Subject: Re: Subgroup analyses - are they ever best evidence Dear Colleagues, Would anyone be prepared to respond to this query from Gayle Robins. All good wishes, Olive >>> "Robins, Gayle" <[log in to unmask]> 09/05/2007 06:01 >>> Hello Mrs Goddard When I evaluate a clinical trial, I use the Oxford Centre for Evidence Based Medicine's recommendation as a guideline for whether the information provided by the trial could be considered as best evidence. I note that individual randomised controlled trials with narrow confidence intervals are considered as level 1b on your levels of evidence chart. Please can you advise me where, if at all, subgroup analyses of these level 1b randomised clinical trials would fall on the best-evidence hierarchy. I realise that there are many different types of subgroup analysis: those that are defined apriori versus retrospectively or on an ad-hoc basis; those that address the same outcome of interest that the randomised controlled trial was designed to assess versus other outcomes; and those that are part of a plethora of subgroup analyses of the same trial and so require correction for multiplicity, to name a few. Are any subgroup analyses of level 1b randomised controlled trials ever considered best evidence? Thanks you for your time taken to read this email. I would appreciate any advice that you can give me, or people that I could contact, in this regard. Gayle Robins Team Leader Clinical Trials Insight Adis International Wolters Kluwer Health Ph: 09 4770700 Email: [log in to unmask]