Not all evidence is judged to be of equal value; that is, there are hierarchies of research design that are evaluated to have different strengths or different levels of value in the decision making process. Of course, the best are at the top and as the pyramids indicate, you have much fewer resources at the top than at the bottom of the pyramids. [Note: The TRIP - or Turning Research Into Practice - Database is freely available at http://www.tripdatabase.com/.]
When you're determining which category an article falls under, there are a few things you should consider. This document will offer tips for assessing the level of evidence in PubMed and provide several examples.