The 6S Hierarchy of Evidence
The 6-S model focuses on appraised literature. Resources at the bottom of the hierachy have not been appraised. Resources from higher on the hierarchy are critically appraised and the evidence is synthesized and often presented in the policy content of a particular country. This model works very well when researching for answers to therapy questions.
[DiCenso, Bayley and Haynes (2009). ACP Journal Club. Editorial: Accessing pre-appraised evidence: Fine-tuning the 5S model into a 6S model. Annals of Internal Medicine, 151(6):JC3-2, JC3-3.]