Types of basis are ranked in five levels. There is a widespread consolidated consensus. In some instances, one randomised trial can be considered sufficient to support this type of evidence. Further confirmatory trials do not seem necessary. Some controlled evidence has therefore been provid ed, but confirmatory trials would be desirable.
Evidence is available from non-randomised studies, with external controls allowing comparisons.
Some uncontrolled evidence has therefore been provided, but trials would be desirable. Little or no direct evidence from clinical studies is available.