Levels of Evidence and Grading Systems

Levels of evidence

A hierarchical system of classifying evidence is an essential part of evidence-based medicine. This hierarchy is commonly referred to as the levels of evidence. When attempting to find the answer to a clinical conundrum, a clinician should actively seek out papers with the highest level of evidence.

The term ‘levels of evidence’ was first used in a report by the Canadian Task Force on the Periodic Health Examination in 1979. The intention was to ‘grade the effectiveness of an intervention according to the quality of evidence obtained’. The Canadian Task Force originally described 4 levels of evidence:

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LEVEL	TYPE OF EVIDENCE
I	At least 1 RCT with proper randomisation
II.1	Well-designed cohort or case-control study
II.2	Time-series comparisons or dramatic results from uncontrolled studies
III	Expert opinions

 

Over the years, many more grading systems have been described, some of the most commonly used being:

• Centre of Evidence-Based Medicine (CEBM), Oxford
• Strength-of-Recommendation Taxonomy (SORT)
• The Jadad scale
• Grading of Recommendations Assessment, Development and Evaluation (GRADE)

Centre of Evidence-Based Medicine

The Centre of Evidence-Based Medicine (CEBM), based at Oxford University, is a not-for-profit organisation that is dedicated to the practice, teaching and dissemination of high-quality, evidence-based medicine that was founded in 1995. The levels of evidence system developed by the CEBM has become one of the most widely used.

The CEBM levels of evidence are as follows:

 

Strength-of-Recommendation Taxonomy

The Strength-of-Recommendation Taxonomy (SORT) is another popular system that was developed by the American Academy of Family Physicians. This system emphasises the use of patient-orientated outcomes that measure changes in morbidity or mortality.

The SORT levels of evidence (codes) are as follows:

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CODE	DEFINITION
A	Consistent, good-quality patient-orientated evidence
B	Inconsistent or limited-quality patient-orientated evidence
C	Consensus, disease-orientated evidence (often expert opinion or case series)

 

The Jadad scale

The Jadad scale, also known as the Oxford quality scoring system, is a means of assessing the quality of the methodology of a clinical trial. The scale is used for a variety of purposes, including:

• To assist with critical appraisal of papers
• As a means of evaluating the quality of a paper
• As an inclusion criterion for including papers in a meta-analysis

The Jadad scale is widely accepted as being a simple and easy to use method for evaluating study methodology. It is considered to be reliable and an excellent means for identifying bias.

The Jadad scale consists of five questions, which are scored as follows:

1. Was the study described as randomised? (+1 Point)
2. Was the study described as double-blind? (+1 Point)
3. Was there a description of withdrawals and dropouts? (+1 Point)
4. Was the method of randomisation described and appropriate? (+1 Point)
5. Was the method of blinding described and appropriate? (+1 Point)

Points are deducted if:

1. The method used to generate the sequence of randomisation was described and it was inappropriate. (-1 Point)
2. The study was described as double-blind, but the method of blinding was inappropriate. (-1 Point)

 

The Jadad scale has received criticism for being too simplistic and placing too much emphasis on blinding. It has also been shown to have a high degree of inconsistency between different raters. Another major criticism is that it does not take into account allocation concealment, which is considered to be of paramount importance by The Cochrane Collaboration.

Grading of Recommendations Assessment, Development and Evaluation (GRADE)

The Grading of Recommendations Assessment, Development and Evaluation (GRADE) working group was founded in 2000 as an informal collaboration of people with an interest in addressing the shortcomings of the present grading systems used in health care.

The GRADE approach is a method of assessing the certainty in evidence and the strength of recommendations in healthcare. It provides a structured and transparent evaluation of the importance of outcomes of alternative management strategies, acknowledgement of patients and the public values and preferences, and comprehensive criteria for downgrading and upgrading certainty in the evidence.