Hierarchy of Design
Strong designs: often trustworthy effects
Multiple randomized controlled trials	The “gold standard” of evidence
Randomized controlled trials	A strong design, but sometimes not feasible
Interrupted time series with a control series	Baseline trends often allow visible effects and controls for biases
Intermediate designs: Sometimes trustworthy effects
Single interrupted time series	Controls for trends, but has no comparison group
Before and after with comparison group (single observations, sometimes called “difference in difference” design)	Comparability of baseline trend often unknown
Weak designs: rarely trustworthy effects (no controls for common biases, excluded from literature syntheses)
Uncontrolled before and after (pre–post)	Simple observations before and after, no baseline trends
Cross-sectional designs	Simple correlation, no baseline, no measure of change