Not all evidence is created equal — so what is good evidence?

Abstract

We are living in the information age, bombarded from every side by bits and bytes of information. How do we know how good any of it is? One of the aims of Evidence-Based Dentistry is to help the practitioner identify the best evidence. Therefore, we identify here the differing levels of evidence and explain how we will be using these in the journal in future.

Main

In our practice, we derive information from a wide array of sources ranging from our own experience to high-quality research. All of this can be described as evidence. Indeed, the essence of the evidence-based approach is to use the evidence from all sources in order to provide the best outcome for the patient. Nevertheless, some evidence is better, stronger or more valid than the rest.

An earlier article¹ highlighted three questions you should ask of each paper:

• Is the study valid?

• What are the results?

• Are the results relevant?

In terms of strength of evidence, it is the validity of a piece of evidence that is important. The validity of a study is the extent to which its design and conduct are likely to prevent systematic errors or bias.² Therefore, the more valid a piece of evidence, the greater its strength and the more secure you can feel making treatment decisions based on it.

The need to develop a method of ranking the validity of evidence was initially developed by Fletcher and Sackett while working on the Canadian Task Force on Periodic Health Examination.³ The result was a table of levels of evidence and related “grades of recommendation” for advice based on the levels of evidence. These initial levels and grades have been widely adopted, often in a slightly modified form, by agencies such as the Scottish Intercollegiate Guideline Network (SIGN) and other bodies that develop guidelines and evidence-based publications.

The initial levels and grades were criticised, however, for their therapeutic/preventive orientation. Consequently, the need to develop similar levels for diagnostic, prognostic, harm and economic studies led a group of people associated with the Centre for Evidence-based Medicine to develop a more complete level-of-evidence table (see Table 1), with associated grades of recommendation (see notes to Table 1). The version printed below was last modified in May 2001, but it is also available on the Centre for Evidence-based Medicine website (www.cebm.net) where it is under constant review, so readers should visit the site from time to time to check for changes.

Table 1 Oxford Centre for Evidence-based Medicine levels of evidence (May 2001).

How we use these levels in the Evidence-Based Dentistry journal

For Evidence-Based Dentistry, we conduct regular searches of the dental and some medical journals to identify possible articles to include in our summary section. The articles we select for inclusion in the journal focus primarily on evidence of level 2a or above although, on occasion, we will include studies down to level 3a evidence as shown in Table 1. Studies below this level are not considered for the journal. The levels of evidence indicated in Table 1 have a narrow focus but, as noted below, the levels may be modified by the addition of plus or minus signs. For example an individual randomised controlled trial with narrow confidence intervals would be rated as a level 1b study. However if the confidence intervals were wide and/or there were other quality questions over the study, this would then be rated level 1b−.

To assist the reader in identifying the level of evidence of a paper, we will in future be including a simple visual device similar to a visual analogue scale. This device will be found at the top of each of the summary papers. An example is shown in Figure 1.

Figure 1.

Examples of graphic device to indicate levels of evidence.