Look at the pyramid below. The evidence provided in an article becomes stronger the higher up it falls on the pyramid.
Levels of evidence are very important to keep in mind when conducting medical research. Don't forget to consider the level of evidence your source provides, no matter where you find it (database, free web, section of a book, etc.). Start at the top of the pyramid for the most reliable evidence, keeping in mind that research may not yet exist or may not be feasible for some questions. Definitions of these articles types can be found here.
