Level 3 – Observational – Analytic Designs
Level 3.a – Systematic review of comparable cohort studies
Level 3.b – Systematic review of comparable cohort and other lower study designs
Level 3.c – Cohort study with control group
Level 3.d – Case – controlled study
Level 3.e – Observational study without a control group
Level 4 – Observational –Descriptive Studies
Level 4.a – Systematic review of descriptive studies