Pe d iatric Ane sth e sia is ind e xe d in Ind e x Me d icus, MEDLINE, Curre nt Co nte nts/Clinical Me d icine , EMBASE/ Exce rp ta Me d ica, Sci Se arch , Re se arch Ale rt, Ad Re f e re nd um Anae sth e sio lo gy, SUBIS and Curre nt Op inio n in Anae sth e sio lo gy Pediatric Anesthesia Volume 22 Supplement 1 July 2012 that the relationship is causal 2+ Well-conducted case-control or cohort studies with a low risk of confounding, bias, or chance and a moderate probability that the relationship is causal 2) Case-control or cohort studies with a high risk of confounding, bias, or chance and a significant risk that the relationship is not causal 3 Non-analytic studies, e.g. case reports, case series 4 Expert opinion 2