Table 1 The Scottish Intercollegiate Guidelines Network (SIGN) scale and the Grading of Recommendations Assessment, Development, and Evaluation (GRADE) scale

SIGN scale

Definition

I +  + 

High-quality meta-analyses, systematic reviews of randomized controlled trials (RCTs), or RCTs with a very low risk of bias

I + 

Well-conducted meta-analyses, systematic reviews of RCTs, or RCTs with a low risk of bias

I-

Meta-analyses, systematic reviews of RCTs, or RCTs with a high risk of bias

II +  + 

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

II + 

Well-conducted case–control or cohort studies with a low risk of confounding or bias and a moderate probability that the relationship is causal

II-

Case–control or cohort studies with a high risk of confounding or bias and a significant risk that the relationship is not causal

III

Nonanalytic studies (e.g., case reports, case series)

GRADE scale

Definition

Good quality

Further research is very unlikely to change our confidence in the estimate of effect

Moderate quality

Further research is likely to have an important impact on our confidence in the estimate of effect and may change the estimate

Insufficient quality

Further research is very likely to have an important impact on our confidence in the estimate of effect and is likely to change the estimate

Any estimate of effect is very uncertain

Strong

recommendation

Used when the desirable effects of an intervention clearly outweigh the undesirable effects or clearly do not

Discretionary

recommendation

Used when the trade-offs are less certain—either because of low-quality evidence or because evidence suggests that desirable and undesirable effects are closely balanced