++High quality meta-analyses, systematic reviews of RCTs, or RCTs with a very low risk of bias 1 + Well conducted meta-analyses, systematic reviews, or RCTs with a low risk of bias 1 -Meta-analyses, systematic reviews, or RCTs with a high risk of bias 2 ++ High quality systematic reviews of case control or cohort studies High quality case control or cohort studies with a very low risk of confounding or bias and a high probability that the relationship is causal 2 + Well conducted case control or cohort studies with a low risk of confounding or bias and a moderate probability that the relationship is causal 2 -Case control or cohort studies with a high risk of confounding or bias and a significant risk that the relationship is not causal 3Non-analytic studies, eg case reports, case series NHS Quality Improvement Scotland (NHS QIS) is committed to equality and diversity and assesses all its publications for likely impact on the six equality groups defined by age, disability, gender, race, religion/belief and sexual orientation.SIGN guidelines are produced using a standard methodology that has been equality impact assessed to ensure that these equality aims are addressed in every guideline. This methodology is set out in the current version of SIGN 50, our guideline manual, which can be found at www.sign.ac.uk/guidelines/fulltext/50/index.html. The EQIA assessment of the manual can be seen at www.sign. ac.uk/pdf/sign50eqia.pdf. The full report in paper form and/or alternative format is available on request from the NHS QIS Equality and Diversity Officer.Every care is taken to ensure that this publication is correct in every detail at the time of publication. However, in the event of errors or omissions corrections will be published in the web version of this document, which is the definitive version at all times. This version can be found on our web site www.sign.ac.uk.This document is produced from elemental chlorine-free material and is sourced from sustainable forests.