SIGN guidelines are developed using an explicit methodology based on three core principles:
Development is carried out by multidisciplinary, nationally representative groups
A systematic review is conducted to identify and critically appraise the evidence
Recommendations are explicitly linked to the supporting evidence.
These principles have remained constant since SIGN was first established.
Although these core principles remain unchanged, detail of how they are reflected in practice has developed over the years. The latest major development has been implementation of the GRADE process. You can see a statement of how SIGN implements the principles of the GRADE process.
As part of the changes related to GRADE, SIGN decided as of 2013 not to continue with the ABCD grading of recommendations that had been in use since 2001. Reasons for this decision are set out in a separate policy document.