IaEvidence form meta-analysis of randomised controlled trials
IbEvidence from at least one randomised controlled trial
IIaEvidence from at least one controlled trial without randomisation
IIbEvidence from at least one other type of quasi-experimental study
IIIEvidence from non-experimental descriptive studies, such as comparative studies, correlation studies and case-control studies
IVEvidence from expert committees reports or opinions and/or clinical experience of respected authorities

From: Appendix D, 2003 guideline appendices

Cover of Infection: Prevention and Control of Healthcare-Associated Infections in Primary and Community Care
Infection: Prevention and Control of Healthcare-Associated Infections in Primary and Community Care: Partial Update of NICE Clinical Guideline 2.
NICE Clinical Guidelines, No. 139.
National Clinical Guideline Centre (UK).
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