Publication types are ranked in evidence pyramids based on the rigor of evidence provided by the research design. The higher the study design is on the pyramid, the higher the level of evidence. However, this also means there are fewer documents of this type.
High level evidence from the top of the pyramid is preferred, but not always available for all questions. Remember that the type of question you are asking will help determine which types of studies would be most appropriate.
Check out the links below for more information of the different levels of evidence and research study designs.