Table 2 Definitions used for the grades of evidence

Grade I: Randomised controlled trial (RCT)

IA Calculation of sample size and accurate, standard definition of outcome variables

IB Accurate and standard definition of outcome variables.

IC Neither of the above

Grade II: Prospective study with a comparison group (non randomised controlled trial, observational study) or retrospective study which controls for confounding variables

IIA Calculation of sample size^a and accurate, standard definition of outcome variables and adjustments for the effects of important confounding variables

IIB One or more of the above

IIC Neither of the above

Grade III Retrospective or observational or cross-sectional studies

IIIA Comparison group, calculation of sample size and accurate, standard definition of outcome variables

IIIB One or more of the above

IIIC Neither of the above.