EB Practitioners must determine the strength of the evidence they find in the literature. Systems for rating the strength of evidence vary widely.
Johns Hopkins Evidence-Based Practice for Nurses and Healthcare Professionals, Fourth Edition by 
Levels of evidence are determined by the following:
1. Strength of Evidence: confidence that the evidence is a true measure
2. Quality of Evidence: minimization of bias
3. Statistical Precision: degree of certainty about the existence of a true measured effect
4. Size of Effect: how much of an effect is above 'No Apparent Effect' for clinically relevant benefits (no harms)
5. Relevance of Evidence: appropriateness of the outcome measure; usefulness in measuring benefits / harms
Highest Level of Evidence -- systematic review of all relevant randomized controlled trials (RCTs).
Next Highest Level of Evidence -- at least one RCT on effectiveness
And so on... e.g. a pseudo-randomized trial that assigns participants by alternating between groups by date of admission
Lower Levels of Evidence --
non-randomized studies with control group running concurrently with intervention group
non-randomized studies with intervention effects which are compared to previous / historical information
single-case studies
Confidence Interval (CI):
Covers the likely range of true effect. (play of chance can skew results)
Take a look at the following websites that offer unique versions of the Evidence Pyramid.