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Online Clinical Library 2: Levels of Evidence

Type of Study and Resources to Use

Systematic Reviews & Meta-analyses
Randomized Controlled Trials
Cohort Studies Case Reports Case Series
Structured Abstracts, Expert topic reviews
Practice Guidelines Consensus Statements
Cochrane Library 1 Cochrane Library

PubMed ACP Journall Club ECRI Guidelines Trust 3
PubMed Clinical Queries PubMed Clinical Queries  CINAHL eMedicine 3 PubMed


DynaMed Plus 4

  1. The Cochrane Library contains several files. The systematic reviews are but one file in their library. Subscription needed.
  2.  CENTRAL: Central Register of Controlled Trials contains bibliographic information on articles retrieved from MEDLINE and EMBASE and other sources. Summaries of articles provided for some. Subscription needed.
  3. Free registration is required.
  4. ACP members have free access to this resource


Systematic Review

The systematic review uses rigorous methods to identify, appraise and synthesize relevant studies, usually double-blinded, randomized controlled clinical trials (RCTs). Meta-analysis is a statistical procedure that combines the results of two or more studies.  Not to be confused with narrative reviews, review articles, review tutorials or literature reviews.

Randomized Controlled Clinical Trial (RCT)

A prospective, analytical, experimental study using primary data generated in the clinical environment. The RCT is the strongest evidence of the clinical efficacy of preventive and therapeutic procedures in the clinical setting.

Cohort Study

A prospective, analytical, observational study based on data from a follow-up period of a group, in which some participants have had, have, or will have the exposure of interest to determine the association between that exposure and the outcome. The observational nature of the cohort study makes it susceptible to bias and the evidence is not as strong as that provided by the RCT.

Case Series

A descriptive, observational study of a series of cases usually describing the manifestations, clinical course and prognosis of a disease or condition. Case series are best used as a source of hypotheses for investigation by stronger study designs. Case series provide weak empirical evidence because of the lack of comparability. Case reports are usually primarily anecdotal evidence.

Structured abstracts, POEMS and Expert Topic Reviews

The structured abstracts of ACP JournalClub, for example and the similarly designed InfoPOEMS (Patient Oriented Evidence that Matters), provides an up to date summary in a structured format of important studies that have a direct relevance to patients. UpToDate and eMedicine are databases of topic reviews, in which expert clinicians or groups of clinicians (editors or editorial boards) appraise the current best research and evidence available and offer their expert, experienced opinion.

Practice Guidelines, consensus statements

In order to provide standardized practice across various patient care settings throughout the country and around the world, various professional associations, government health agencies have designated guidelines, usually based on the best available evidence, to establish standards for care. These standards may be local and are continuously updated to reflect current scientific research and professional practice.