Level I – Systematic Reviews, Meta-Analyses, Randomized Control Trials
Level II – Two groups, non-randomized studies ( e.g., cohort, case control, pre/post-test)
Level III – One group, nonrandomized studies (e.g., before and after, pre/post-test, time series, longitudinal studies)
Level IV – Descriptive studies that include analysis of outcomes (single subject design, case series, case study, survey studies)
Level V – Expert opinion, literature reviews, research not performed on human subjects
Hissong, A.N., Lape, J. E., & Bailey, D. M. (2015). Bailey’s research for the health professional. (3rd ed.). Philadelphia, PA FA Davis.