Introduction: Actors in health systems make decisions that should be based on the best available evidence. This raises important issues of what is evidence, how can it be evaluated and graded. Numerous quality scales and checklists have been developed, some of which aim to quantify the level of evidence. However, several limitations restrict the use of these scales for the assessment of the level of evidence.

First, the level of evidence for an individual clinical trial is sometimes incorporated with the level of evidence for a systematic review of several trials.

Second, none of the existing scales include all three of the following components

* the appropriateness of the trial design to show an unbiased measure of the treatment effect;
* the quality of the collected data in compliance with the trial's objective (i.e. the hypothesis tested); and
* the relevance of the hypothesis, outcomes used and the population studied to the clinical problem.


Third, there is no general agreement about the weight to be given to each item. Hence the level of evidence assessment should include a dimension related to the nature of the hypothesis tested and also refer to a system of values. What has been proved and the process by which this was achieved (both theoretical i.e. methods, and practical i.e. execution) should be considered when judging the level of evidence.

Methods:

Using the various published scales we have identified criteria which may be used for judging the level of evidence, without considering their respective weight. A panel of experts is being constituted and this panel will give their opinion on the level of evidence for a series of clinical trials. We will then model the weights accorded by the experts to our criteria, and using multi-criteria methods (commonly used in health economics research) we will define the hierarchical organisation. A parallel approach will examine the epistemological and sociological aspects, since the nature of the evidence depends on the hypothesis formulated, the means used to verify it, the interpretation of the results, and their generalisation. We will investigate if it is possible to use a scale to determine the level of evidence, since values vary under different circumstances and can change over time.