Table 2Hierarchy of study designs

Level 1. Randomized controlled trials
Level 2. Controlled Before-After (CBA) —contemporaneous observation periods for control and intervention groups before and after an intervention
Interrupted time series (ITS)—well-defined time period for intervention implementation and at least three time points both before and after intervention
Quasi-randomized trials—contained at least two cohorts of patients assembled prospectively based on an allocation procedure that was non-random, but arbitrary, in the sense of bearing no apparent connection to patient or provider factors that might affect intervention outcome (e.g. alternation, date of birth, even/odd character of provider or patient identification)
Level 3. Observational studies with controls—includes before-after and time series not meeting strict definitions of CBA and ITS (above), case-control studies, cohort studies with controls.
Level 4. Observational studies without controls (e.g., cohort studies without controls and case series)

From: 2, Evidence-based Review Methodology for the Closing the Quality Gap Series

Cover of Closing the Quality Gap: A Critical Analysis of Quality Improvement Strategies (Vol. 1: Series Overview and Methodology)
Closing the Quality Gap: A Critical Analysis of Quality Improvement Strategies (Vol. 1: Series Overview and Methodology).
Technical Reviews, No. 9.1.
Shojania KG, McDonald KM, Wachter RM, et al., editors.

NCBI Bookshelf. A service of the National Library of Medicine, National Institutes of Health.