Methods in health services research: Interpreting the evidence: choosing between randomised and non-randomised studies

BMJ 1999;319:312-315 ( 31 July )

Education and debate

Methods in health services research

Interpreting the evidence: choosing between randomised and non-randomised studies

This is the first of four articles

Martin McKee, professor of European public health ^a, Annie Britton, research fellow ^a, Nick Black, professor of health services research ^a, Klim McPherson, professor of public health epidemiology ^a, Colin Sanderson, senior lecturer in health services research ^a, Chris Bain, reader in social and preventive medicine ^b.

^a London School of Hygiene and Tropical Medicine, London WC1E 7HT, ^b University of Queensland Medical School, Brisbane 4006, Australia

Correspondence to: Martin McKeem.mckee@lshtm.ac.uk

The first 150 words of the full text of this article appear below.

Evaluations of healthcare interventions can either randomise subjects to comparison groups, or not. In both designs thereare potential threats to validity, which can be external (theextent to which they are generalisable to all potential recipients)or internal (whether differences in observed effects can be attributedto differences in the intervention). Randomisation should ensurethat comparison groups of sufficient size differ only in theirexposure to the intervention concerned. However, some investigatorshave argued that randomised controlled trials (RCTs) tend to exclude,consciously or otherwise, some types of patient to whom resultswill subsequently be applied. Furthermore, in unblinded trialsthe outcome of treatment may be influenced by practitioners' andpatients' preferences for one or other intervention. Though non-randomisedstudies are less selective in terms of recruitment, they are subjectto selection bias in allocation if treatment is related to initialprognosis.

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These issues have led to extensive debate, . . . [Full text of this article]

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Rapid Responses:

Read all Rapid Responses

Two systematic reviews-two different answers: Benjamin Djulbegovic
bmj.com, 18 Aug 1999 [Full text]
Two systematic reviews-two different answers?: Regina Kunz, et al.
bmj.com, 28 Aug 1999 [Full text]
Three systematic reviews - not so different answers?: Annie Britton, et al.
bmj.com, 29 Sep 1999 [Full text]