Table 1 Levels of evidence used in the best-evidence synthesis

Strong

Generally consistent (non)significant associations found in at least two good-quality cohort studies

Moderate

Generally consistent (non)significant associations in one good-quality cohort study and at least one fair-quality cohort study

Weak

(Non)significant association found in one good-quality or fair-quality study or generally consistent (non)significant associations found in at least two poor-quality studies

Inconclusive

(Non)significant association found in less than two poor-quality studies

Inconsistent

Inconsistent significant findings irrespective of study quality

(i.e. <66% of the studies reported consistent findings)