| Level I | Good quality evidence |
Systematic review of RCTs with consistent findings High quality individual RCT |
| Level II |
Limited quality patient orientated evidence |
Systematic review of lower quality studies or studies with inconsistent findings Low quality clinical trial Cohort studies Case-control studies |
| Level III | Other |
Consensus guidelines, extrapolations from bench research, usual practice, opinion, disease-oriented evidence (intermediate or physiologic outcomes only), or case series |
[14] Retinoids: Heckel S, Favre R, Weber P, Dellenbach P. [Teratogenicity of retinoids. A case and review of the literature]. J Gynecol Obstet Biol Reprod (Paris). 1993;22(1):43-7.https://www.ncbi.nlm.nih.gov/pubmed/8463566
Retinoids: Jick SS, Terris BZ, Jick H. First trimester topical tretinoin and congenital disorders. Lancet. 1993 May 8;341(8854):1181–2. https://www.ncbi.nlm.nih.gov/pubmed/8098078