Table 5.

Quality of included studies (n=235).

Level of evidencea	Studies, n (%)
Level 1	7 (3)
Level 2	30 (12.8)
Level 3	45 (19.1)
Level 4	63 (26.8)
Level 5	36 (15.3)
Unable to classify (narrative reviews)	54 (22.9)

^aLevel 1 studies represent the highest level of evidence and are reserved for systematic reviews; a randomized controlled trial is classified as a level 2 study; nonrandomized, cross-sectional and cohort studies are classified as level 3; level 4 studies are case-series or case-control studies; and studies conducted using animal models of disease are classified as level 5—the lowest-quality evidence.