Reporting levels of evidence is an important part of critical appraisal.
In Australia NHMRC Levels of Evidence are used. However institutions around the world may use other levels of evidence. Always cite the source of the level of evidence you are using. See the following PowerPoint for an overview of available levels of evidence:
It usually comes as a surprise to students to learn that some (the purists would say 99% of) published articles belong in the bin, and should certainly not be used to inform practice". Greenhalgh, p.31 2010.