Table 1.

Classic levels of evidence for rehabilitation practice. Table reprinted in part with the permission of SLACK Inc.[24]

LEVEL	CLASSIC "LEVELS OF EVIDENCE” FOR THERAPY/ PREVENTION	PLACEMENT OF ADDITIONAL TYPES OF CLINICAL EVIDENCE
1a	Systematic review (with homogeneity) of randomized controlled trials (RCT)	Clinical practice guidelines (CPG's) where recommendations are based on systematic reviews that contain multiple RCTs and the development includes supplemental data or expert opinion to make recommendations only where evidence is lacking
1b	Individual RCT (with narrow Confidence Interval)
1c	All or none
2a	Systematic review (with homogeneity) of cohort studies	Lower quality CPGs that are based on informal evidence review and expert consensus, where few RCT's are identified
2b	Individual cohort study (including low quality RCT; e.g., <80% follow-up)
2c	“Outcomes” research
3a	Systematic review (with homogeneity) of case-control studies	Structured consensus processes based on quantitative ratings of agreement and formal consensus processes using qualified experts
3b	Individual case-control study
4	Case-series (and poor quality cohort and case-control studies)	Unstructured quantitative or qualitative expert consensus; large descriptive practice analysis/survey that defines common ground; critical appraisal/comprehensive systematic review or synthesis of biologic studies, or first principles
5	Expert opinion without explicit critical appraisal, or based on physiology, bench research, or “first principles.”	CPG's that are not based on the use of evidence review or quantitative data; clinical protocols or rehabilitation theory