Level of evidence: how sure we are that the identified evidence is a true measure of the benefits of an intervention.
The study design used in Cochrane reviews, a systematic review of all relevant randomised controlled trials, is the highest level of evidence. The next level is provided by at least one randomised controlled trial on the effectiveness of an intervention, then a pseudo-randomised trial e.g. a trial that assigns participants to a treatment group by alternating between groups as they present or by date of admission.
Quality of evidence: determined by the methods used to minimise bias within a study design.
Statistical precision: the degree of certainty about the existence of a true measured effect.
2. Size of effect
How much the determined intervention effect is above a ‘no apparent effect’ value for clinically relevant benefits (or harms).
3. Relevance of the evidence
How appropriate the outcome measure is for the healthcare problem, and its usefulness in measuring the benefits (or harms) of the treatment.
4. Lower levels of evidence are provided by:
non-randomised studies of groups of people where a control group has run concurrently with the group receiving the intervention being assessed;
non-randomised studies of groups of people where intervention effects are compared with previous or historical information;
single case studies.
Confidence interval (CI):
Even studies that are perfectly designed and carried out may show variable results because of the play of chance. CI covers the likely range of the true effect. For example, the result of a study may be that 40% (95% CI 30% to 50%) of people are helped by a treatment. That means that we can be 95% certain the true effect is between 30% and 50%.
(From: Smart Health Choices How to make informed health decisions by Judy Irwig, Les Irwig and Melissa Sweet, Allen and Unwin 1999)
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