This is the "Levels of Evidence" page of the "Evidence-based Clinical Practice" guide.
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Evidence-based Clinical Practice   Tags: databases, evidence, evidence-based  

A guide to evidence-based chiropractic resources at Palmer and on the web.
Last Updated: May 9, 2017 URL: Print Guide RSS UpdatesEmail Alerts

Levels of Evidence Print Page


This page explains how clinicians determine the best evidence.


Evidence & Databases

Different databases contain different kinds of evidence. See the Databases tabs in this guide to find which databases have the evidence you need.


Levels of Evidence

Categories of evidence are conceptualized as a hierarchy, with the top levels considered the best evidence for clinical decision-making. These higher levels are considered secondary literature, which synthesizes, filters, and evaluates the primary (lower level) literature. The hierarchy can include:

  • Systematic Reviews: A review of all research available on a given topic, conducted with strict methods of locating and synthesizing the research.
  • Systematic Reviews with Meta-Analyses pool and summarize the data from each study included in the review.
  • Critically-Appraised Topics: A critical appraisal of clinically relevant studies.
  • Critically-Appraised Individual Articles: Articles that are selected and rated for clinical relevance by physicians.
  • Randomized Controlled Trials: A study wherein the subjects are divided into two groups: one that received the genuine treatment, and one that receives a placebo treatment.
  • Cohort Studies: A study that compares outcomes between two groups studied over a period of time.
  • Case-Controlled Studies: A study that compares groups with differing outcomes with respect to their exposure to a suspected causal attribute.
  • Case Series: A study that measures the outcome of a group of patients who receive the same treatment, with no control group.
  • Case Reports: Reports on the treatment and outcome of a single patient.

pyramid showing levels of evidence

More Resources for Levels of Evidence

  • Forest Plots: Trying to See the Wood and the Trees
    A simple explanation of how meta-analyses use forest plots to summarize multiple sets of data. Lewis S, Clarke M. Forest plots: trying to see the wood and the trees. BMJ : British Medical Journal. 2001;322(7300):1479-1480.
  • Of Studies, Syntheses, Synopses, and Systems
    Haynes RB. Of Studies, Syntheses, Synopses, and Systems: the "4S" Evolution of Services for Finding Current Best Evidence. Evid Based Med 2001; 6(2): 36-38.

Highlighted Resource

Cover Art
How to Read a Paper: the Basics of Evidence-Based Medicine - Greenhalgh, Tricia.
Call Number: R 118.6 .G77 2006
ISBN: 1405139765
Publication Date: 2006


Website Disclaimer


Palmer College of Chiropractic is not responsible for the content of the external websites contained in this research guide. The views and opinions of the authors expressed in these external websites do not necessarily state or reflect those of Palmer College of Chiropractic.

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