This document discusses different frameworks for organizing levels of evidence from healthcare research. It references the "4S", "5S", and "6S" hierarchies, which provide a structure for categorizing different types of clinical research studies from most to least rigorous based on their methodology and potential for bias. The frameworks help practitioners identify the strongest available evidence when evaluating research findings and their implications for clinical practice.

1. “ 4S” levels of organisation of evidence from research Haynes B Evid Based Nurs 2005; 8:4-6 ©2005 by BMJ Publishing Group Ltd and RCN Publishing Company Ltd

2. The “5S” levels of organisation of evidence from healthcare research. Haynes B Evid Based Nurs 2007;10:6-7 ©2007 by BMJ Publishing Group Ltd and RCN Publishing Company Ltd

3. The 6S hierarchy of pre-appraised evidence DiCenso A et al. Evid Based Nurs 2009;12:99-101 ©2009 by BMJ Publishing Group Ltd and RCN Publishing Company Ltd