Table 1

Levels of evidence

EvidenceClassRequirement
ConvincingClass INumber of cases >1000, meta-analysis p value <10−6, between study heterogeneity I2 <50% and 95% prediction interval excluding the null; no detected publication bias
Highly suggestiveClass IINumber of cases >1000, meta-analysis p value <10−6, the largest study with a statistically significant effect and class I criteria not meet
SuggestiveClass IIINumber of cases> 1000, meta-analysis p value <10−3 and class I or II criteria not meet
WeakClass IVMeta-analysis p value <0.05 and classes I–III criteria not meet
Non-significantNullmeta-analysis p value> 0.05