Table 2 Levels of evidence (adapted from the Oxford 2011 Levels of Evidence52)

From: A review of research trends in physiological abnormalities in autism spectrum disorders: immune dysregulation, inflammation, oxidative stress, mitochondrial dysfunction and environmental toxicant exposures

Level Description
1 SR of RCT; individual RCT (with narrow confidence intervals); cohort (prospective) study with good follow –up.
2 SR of cohort studies with homogeneity; individual cohort (prospective) study; low-quality RCT.
3 SR of case-control studies with homogeneity; individual case-control (retrospective) study.
4 Case series or reports.
5 Expert opinion without critical appraisal.
  1. Abbreviations: RCT, randomized controlled trial, SR, systematic review.