Level of evidenceStudy results(a)
Strong evidence for an inverse association≥75% of all high-quality studies(b) reporting a decreased risk,(c) or
≥60-75% of all high-quality studies reporting a decreased risk and <10% of all high-quality studies reporting an increased risk(c)
Moderate evidence for an inverse association≥60-75% of high-quality studies reporting a decreased risk and <25% of all high-quality studies reporting an increased risk, or
≥50-60% of all high-quality studies reporting a decreased risk and <10% of all high-quality studies reporting an increased risk
Indecisive evidenceinconsistent findings defined as all other findings not applicable to strong, moderate or no evidence for an inverse association, or there are <4 high-quality studies available
Nil≥60% of all high-quality studies reporting no association or an increased risk, or
≥40% of all high-quality studies reporting an increased risk